I apologize in advance for the huge wall of text. I actually had to trim this down quite a bit to fit under the 30,000 character limit!

My own research and analysis

The first revelation (at least in the sequence they are presented in The Poem of the Man-God) to deal with God's creation of humans is part of the section titled Birth of the Virgin Mary. In particular, Jesus speaks to Maria Valtorta of this in a revelation given on Sunday August 27th, 1944. For the sake of convenience, I will refer to this as "Passage 1".

The revelation which relates to original sin is given in the context of clarifying the profound significance of the Annunciation and the Virgin Mary's role in God's plan for Salvation. The Poem of the Man-God reports this revelation as having been given on Sunday March 5th, 1944, which was the Second Sunday of Lent. The passage is titled The Disobedience of Eve and the Obedience of Mary, which (for the sake of convenience) I will refer to as "Passage 2".

Jesus brings up the topic of original sin in Passage 2.

Jesus says: «To make you understand how grateful you must be to Him Who died to elevate you to Heaven and to defeat Satan's concupiscence, I wanted to speak to you, in this period of preparation for Easter, of what was the first link of the chain by which the Word of the Father was dragged to death, the Divine Lamb to the slaughterhouse. I wanted to speak to you about it, because at present ninety per cent of you are like Eve intoxicated by Lucifer's breath and words, and you do not live, to love one another, but to glut yourselves with sensuality, you do not live for Heaven but for filth, you are no longer creatures gifted with soul and reason, but dogs without soul and without reason. You have killed your souls and perverted your reason. I solemnly tell you that brutes surpass you in the honesty of their love.

...Do we not read in Genesis that God made man the overlord of everything on the earth, that is everything except God and His angelical ministers? Do we not read that He made the woman the companion of man in his joy and his domination over all living beings? Do we not read that they were allowed to eat of everything with the exception of the tree of the knowledge of Good and Evil? Why? What is the meaning of the words "that he might rule"? And what is the meaning of the tree of the knowledge of Good and Evil? Have you ever asked these questions, you man, who ask so many useless ones and never ask your soul about heavenly truths?»

Why did God create humans?

Genesis 1:27 --

So God created humankind in his image, in the image of God he created them; male and female he created them.

Passage 1 --

Jesus says: «God desired to put a king in the universe that He had created out of nothing. A king, who by the nature of matter should be the first amongst all the creatures created with matter and endowed with matter. A king, who by nature of the spirit should be little less than divine, united to Grace as he was in his first innocent day. But the Supreme Mind, to Whom all the most remote events in centuries are known, incessantly sees what was, is and will be; and while It contemplates the past, and observes the present, It penetrates deeply with Its foresight into the most distant future and knows in every detail how the last man will die. Without confusion or discontinuity the Supreme Mind has always known that the king created to be demigod at Its side in Heaven, heir of the Father, would arrive adult in His Kingdom, after living in the house of his mother -the earth, with which he was made- during his childhood, as child of the Eternal Father for his day on earth...»

Passage 2 --

Jesus says: «How many truths your soul would tell you, if you only knew how to converse with it, if you loved your soul that makes you like God, Who is a spirit, as your soul is a spirit.»

God has created humans to be the crowning jewel of Creation. Having a soul, Man is the one thing on the Earth God has created that is most like Himself. That being the case, it seems only fitting that Man should be permitted to rule over the Earth.

What does it mean for God to give dominion to humans?

Genesis 1:26 --

... and let them have dominion over the fish of the sea, and over the birds of the air, and over the cattle, and over all the wild animals of the earth, and over every creeping thing that creeps upon the earth.”

Passage 2 --

Jesus says: «If you knew how to question your soul, you would be told that the true, extensive meaning -as comprehensive as creation itself- of the words "that he might rule" is this:

"That man might dominate everything, that is his three states. The lower state, the animal one. The middle state, the moral one. The superior state, the spiritual one. And all three of them are to be directed to one sole aim: to possess God".

To possess Him by deserving Him through a strict control which subdues all the power of one's ego and conveys it to one only purpose: to deserve to possess God.»

Apparently man's dominion over the Earth includes a sort of self-mastery. It seems to me the implication is that free will is a necessary prerequisite for man's dominion over the Earth to truly be considered complete, since mastery of the self means having control over our own decisions and actions.

Where did evil come from?

Passage 2 --

Jesus says: «Lucifer was an angel, the most beautiful of all the angels, a perfect spirit, inferior only to God, and yet in his bright essence a vapour of pride arose and he did not scatter it. On the contrary, he condensed it by brooding over it. And Evil was born of this incubation. It existed before man. God had hurled him out of Paradise, the cursed incubator of Evil, who had desecrated Paradise. But he is the eternal incubator of Evil and as he can no longer soil Paradise, he has soiled the earth.»

According to the revelation, Satan's pride is the source of evil.

This particular excerpt, I think, has great importance in that it explicitly identifies the Serpent as Satan. Although , the identity of the Serpent is only implied to be Satan in Biblical verses like Wisdom 2:24 and Revelation 12:9, and titles applied to him like "The Tempter" in Matthew 4:3 and "The Ancient Serpent" in Revelation 20:2.

Additionally, this excerpt explicitly identifies Pride as the root of all evil, justifying classifications of Pride as the most foul of the Seven Deadly Sins.

Why did God forbid Adam and Eve to eat of the Tree of Knowledge?

Genesis 2:16 --

And the Lord God commanded the man, “You may freely eat of every tree of the garden; 17 but of the tree of the knowledge of good and evil you shall not eat, for in the day that you eat of it you shall die.”

Passage 2 --

Jesus says: «Your soul would tell you that God had forbidden the knowledge of good and evil, because He had already granted good to His creatures gratuitously, and He did not want you to know evil, because it is a sweet fruit to taste, but once its juice becomes part of your blood, it causes a fever that kills you and produces a parching thirst, so that the more one drinks of that false juice, the more thirsty one becomes...

Also Eve knew what was good for her to know. But not more, because it is valueless to know what is not good.»

God has already given Adam and Eve each other's companionship. God has given them dominion and made them King and Queen of the Earth. He has even made them in His own image; Adam and Eve are -- in almost every way -- already like God, who is Perfect.

The reason for God's command not to eat of the Tree of Knowledge of Good and Evil becomes clear: God has already given to Adam and Eve everything that is Good; indeed "every tree in the garden"! The only thing that God has kept from them is Evil -- and even knowledge of what Evil is -- because Evil has no value, and God does not give gifts which have no value.

This is supported by the following from Passage 1:

Jesus says: «He treats you as if you were good children, who are taught and granted everything so that their lives might be happier and more pleasant. What you know, you know by the light of God. What you discover, you discover through the guidance of God. In Goodness. Other knowledge and discoveries that bear the mark of evil, come from the Supreme Evil: Satan.»

Why did God put the Tree of Knowledge in the Garden of Eden in the first place?

Passage 2 --

Jesus says: «You may object: "And why did He put it there?" Because evil is a force that originated by itself like certain monstrous diseases in the most wholesome body.»

«I said: "metaphorical tree". Now I will say: "symbolical tree". Perhaps you will understand better. Its symbol is clear: the inclination to good and to evil of the two children of God, would be understood by their behaviour towards the tree. Like 'aqua regia' that tests gold and the scales of the goldsmith that weigh its carats, that tree, by God's command, became a means of testing and it gave the measure of Adam's and Eve's symbolic metal purity.»

As we already know, God is Infinitely Just. Having already received so much from God, Adam and Eve must prove themselves worthy of the may gifts God has bestowed upon them in His Infinite Magnanimity. As previously established, Evil has already been brought about by the pride of Satan, but it has not yet corrupted the Earth.

The Tree of Knowledge is metaphorical; there is no literal fruit for them to eat. The test of Adam and Eve, the command which God gives them -- the only command that God gives them -- seems to be that they are not to ask God for more than what He has already given them (viz. everything that is Good).

Inded, the statement "they were not to eat (viz. ask/take for themselves) of the Tree of Knowledge" would seem better expressed as "the act itself of eating (viz. asking/taking for themselves) was the Tree of Knowledge".

A revelation from the Virgin Mary in Passage 2 supports this idea:

Mary says: «The tree: the means to test their obedience. What does obedience to God's commands imply? It implies all possible good, because God commands nothing but good. What is disobedience? It is evil, because it brings about a rebellious mental state in which Satan can be active.»

What exactly was the Fruit of the Tree of Knowledge?

If God made Adam and Eve in His Image and gave them all that was Good, what was left that distinguished God from Man?

One of the distinguishing factors was obvious: Adam and Eve were creatures with flesh and physical form; God is Spirit. But physical form is something that Adam and Eve already had.

The only thing remaining that distinguished them was that Adam and Eve were created by God, whereas God is Eternal, Infinite and Uncreated. Before the Fall of Man, God was the sole Creator of human life.

Passage 2 --

Jesus says: «That metaphorical tree proves this truth. God had said to the man and the woman: "You know all the laws and the mysteries of creation. But do not infringe on My right of being the Creator of man... I have given you everything. I am only keeping for Myself this mystery of the formation of man".»

What is the meaning of the symbolic eating of the fruit?

Genesis 3:1-7

Now the serpent was more crafty than any other wild animal that the Lord God had made. He said to the woman, “Did God say, ‘You shall not eat from any tree in the garden’?” The woman said to the serpent, “We may eat of the fruit of the trees in the garden; but God said, ‘You shall not eat of the fruit of the tree that is in the middle of the garden, nor shall you touch it, or you shall die.’” But the serpent said to the woman, “You will not die; for God knows that when you eat of it your eyes will be opened, and you will be like God, knowing good and evil.” So when the woman saw that the tree was good for food, and that it was a delight to the eyes, and that the tree was to be desired to make one wise, she took of its fruit and ate; and she also gave some to her husband, who was with her, and he ate. Then the eyes of both were opened, and they knew that they were naked; and they sewed fig leaves together and made loincloths for themselves.

To eat the fruit of the tree can mean only one thing: to take the one thing that God has reserved for Himself -- the knowledge and act of creating human life -- and to do it without Him or His involvement. But Adam and Eve went one step further...

The previous excerpt, unabridged, reads (emphasis mine):

«That metaphorical tree proves this truth. God had said to the man and the woman: "You know all the laws and the mysteries of creation. But do not infringe on My right of being the Creator of man. My love will suffice for the propagation of the human race and it will spread among you and will excite the new Adams of the race without any lust of the senses but with purely charitable pulsations. I have given you everything. I am only keeping for Myself this mystery of the formation of man".»

From other excerpts, it appears that sexual activity itself was not what went against God's Plan, but the fact that the first man and the first woman carried out the act with lust in their hearts. The act was performed not for the purpose of creating life, but to satisfy the craving for sensual pleasures. The creation of life became a mere afterthought, and in this the act of creating was profaned.

The passages further clarify that not only was the knowledge of how to create life something God meant to keep for Himself (or at the very least something that He planned to reveal to Adam and Eve at a time and in a manner that He pleased), but that the act of creation and the multiplication of human life was intended as a means for the multiplication of love. Not the sensual or erotic things that we humans often confuse with love, but a love that would be deep and true and holy; a love that would be a reflection of God's Infinite and Perfect Love for his creatures, worthy of being called "love"!

Passage 1 --

Jesus says: «God, the Father Creator, had created man and woman with such a perfect law of love that you cannot even understand its perfection any longer. And you become lost in wondering how the human species would have come to be, if man had not been taught by Satan how to obtain it.

Look at the fruit and seed plants. Do they produce seed and fruit by means of fornication, by means of one fecundation out of one hundred copulations? No. The pollen emerges from the male flower and driven by a complex of meteoric and magnetic laws it proceeds to the ovary of the female flower. The latter opens, receives it and produces. It does not pollute itself and then refuse it, as you do, to enjoy the same sensation the following day. It produces and until the new season, it does not get pollinated and when it does, it is only to produce.

Look at the animals. All of them. Have you ever seen a male animal and a female one approach each other for a sterile embrace and lascivious dealings? No. From near or far, they fly, crawl, jump or run, they go, when it is time, to the fecundation rite. Neither do they evade stopping at the pleasure, but they go further, to the serious and holy consequences of the offspring, the only reason that should cause a man, a demigod by his origin of Grace which I have made complete, to accept the animality of the act, necessary since you descended by one degree towards animals.

You do not act as plants and animals do. You had as your teacher Satan. You wanted him as your teacher and you still want him. And the works you do are what one would expect of the teacher you wanted. Had you been faithful to God, you would have had the joy of children, in a holy way, without pain, without exhausting yourselves in obscene and shameful intercourses, which even beasts are unacquainted with, although beasts are without a reasoning and spiritual soul.»

Passage 2 --

Jesus says: «Satan wanted to deprive man of this intellectual virginity and with his venomous tongue he blandished and caressed Eve's limbs and eyes, exciting reflections and a perspicacity which they did not have before, because malice had not yet intoxicated them.

She "saw". And seeing, she wanted to try. Her flesh was aroused. Oh! If she had called to God! If she had hurried to Him saying: "Father! The Serpent has caressed me and I am upset". The Father would have purified and healed her with His breath, which could have infused new innocence into her as it had infused life. And it would have made her forget the snake's poison, nay it would have engendered in her a disgust for the Serpent, as it happens in those who bear an instinctive dislike for diseases of which they have just been cured. But Eve does not go to the Father. Eve goes back to the Serpent. The sensation is a sweet one for her. "Seeing that the fruit of the tree was good to eat and pleasing and agreeable to the eye, she took it and ate it".

And "she understood". Now Malice was inside her and was gnawing at her intestines. She saw with new eyes and heard with new ears the habits and voices of beasts. And she craved for them with insane greed.

She began the sin by herself. She accomplished it with her companion. That is why a heavier sentence is laid on woman. Because of her, man has become rebellious towards God and has become acquainted with lewdness and death. Because of her, he was no longer capable of dominating his three reigns: the reign of the spirit, because he allowed the spirit to disobey God; the moral reign, because he allowed passions to master him; the reign of the flesh, because he lowered it down to the instinctive level of beasts. "The Serpent seduced me" says Eve. "The woman offered me the fruit and I ate of it" says Adam. And the triple greed has ruled the three dominions since then.»

The Virgin Mary, too, gives explanation in Passage 2.

Mary says: «The Spirit of God had informed Me of the pain of the Father because of the corruption of Eve, who had lowered herself to the level of inferior creatures, whereas she was a creature of grace...

Disobedience was the beginning of the downfall: "Do not eat and do not touch of that tree" said God. And man and woman did not respect that prohibition, although as kings of creation they were allowed to touch and eat of everything except of that tree because God wanted them to be inferior only to angels.

The tree: the means to test their obedience. What does obedience to God's commands imply? It implies all possible good, because God commands nothing but good. What is disobedience? It is evil, because it brings about a rebellious mental state in which Satan can be active.

Eve goes toward the tree, which, if avoided, would have caused her welfare, if approached, would cause her ruin. She goes there led by the childish curiosity of seeing what is special about it, and by a rashness that makes her consider God's command a useless one since she is strong and pure, the queen of Eden, where everything is subject to her and nothing can hurt her. Her presumption is her ruin. Presumption is the yeast of pride.

At the tree she finds the Seducer, who sings his song of lies to her inexperience, to her beautiful virginal inexperience, to her badly guarded inexperience. "You think there is evil here? No, there isn't. God told you because He wants to keep you as slaves under His power. You think you are king and queen? You are not even as free as wild animals. Animals can love one another with true love. You cannot. Animals are granted the gift of being creators like God. Animals generate little ones and see their families grow as much as they like. You do not. You are denied this joy. Why make you man and woman if you have to live thus? Be gods. You do not know the joy of being two in one flesh, that creates a third one and many more. Do not believe God when He promised you the joy of posterity seeing your children forming new families, leaving their father and mother for their families. He has given you a sham life: real life is to know the laws of life. Then you will be like gods and will be able to say to God: 'We are equal to You' ".

And the allurement continued because there was no will to break it, on the contrary there was the will to continue it and to learn what did not belong to man. And the forbidden tree becomes really mortal for the human race because from its branches there hangs the fruit of bitter knowledge that comes from Satan. And the woman becomes a female and with the yeast of Satanic knowledge in her heart, she moves on to corrupt Adam. With their bodies and souls degraded and their morals corrupted, they became acquainted with sorrow and the death of both their souls deprived of Grace and of their bodies divested of immortality. And Eve's wound engendered suffering, which will not subside until the last couple on earth are dead.»

Jesus continues (bold emphasis mine):

«Also the fruit that was offered and eaten was symbolical. It was the fruit of an experience they wanted to have at Satan's instigation to break God's command. I had not forbidden men love. I only wanted them to love each other without malice; as I loved them in My holiness, they were to love each other in the holiness of affections unsoiled by lewdness...

But she [Eve] did not have faith in God's word, and was not faithful to her promise of obedience. She believed in Satan, she broke her promise, she wanted to know what was not good, she loved it without regret, she turned love into something corrupt and degraded, which I instead had permitted as something holy. A sullied angel, she wallowed in mud and litter, whereas she could have run happily amongst the flowers of the earthly Paradise and she could have seen her offspring flourish around her, like a plant that is covered with flowers without bending its leaves into the mire.»

Jesus goes on to explain that, by this first sin, Man permitted misery to enter the world:

«...o Cursed One, who deprived God of the joy of being the Father of all men created!

In vain you have corrupted them, who had been created innocent, leading them to knowledge and conception by means of the sensuousness of lust, depriving God, in His beloved creature, of being the benefactor of the children according to rules, which, had they been respected, would have kept a balance on earth between sexes and races, a balance capable of averting wars between peoples and calamities between families.

By obeying, they would have also known love. Nay, only by obeying they would have known love and possessed it. A complete and peaceful possession of this gift from God, Who from the supernatural descends to the inferior, so that also the flesh may rejoice devoutly, since it is united to the spirit and created by Him Who created the spirit.

Now, men, what is your love, what are your loves? Either lewdness disguised as love or an incurable fear of losing the love of your partner through her or other people's lewdness. You are never sure of possessing the heart of your husband or wife, since lust entered the world. And you tremble and cry and become overwrought with jealousy, sometimes you kill to avenge a betrayal, sometimes you despair, and sometimes you lack will or even become insane.

This is what you have done, Satan, to the children of God. Those whom you have corrupted, would have known the joy of having children without suffering any pain and would have experienced the joy of being born without fear of dying.»

What does this mean for us, the descendants of Adam and Eve?

As the Church teaches, we have inherited the stain of original sin. However, Jesus clarifies that although we bear the consequences of original sin (which are dissolved through Baptism), we do not share in Adam and Eve's original guilt:

«I can already hear your objection: "Was the punishment not excessive and the means used to condemn them not childish?"

Not so. Actual disobedience in you, who are their heirs, is not so grave as if it were in them. You have been redeemed by Me. But Satan's poison is always ready to rise again, like certain diseases that never disappear completely in the blood. The First Parents possessed Grace without ever even nearing Disgrace. They were therefore stronger and more firmly supported by Grace that generated love and innocence. The gift given them by God was infinite. Much graver is therefore their fall notwithstanding that gift.»

In effect, our disobedience to God when we sin (as bad as it is) is still not as bad as the disobedience of Adam and Eve since -- never having risen as high as them -- we have a much shorter distance to fall.