What is Adam, if he isn't literally the first human?

I hope I understand your question correctly, in that you find confusion in the part about Adam introducing sin into the World; and the part about death reigning over all people until Moses introduced the LAW. First of all we need understand Adam's disobedience. Adam's sin of disobedience was Intentional.

Genesis 3:6 KJV  And when the woman saw that the tree was good for food, and that it was pleasant to the eyes, and a tree to be desired to make one wise, she took of the fruit thereof, and did eat, and gave also unto her husband with her; and he did eat.

Gen 3:17 KJV  And unto Adam he said, Because thou hast hearkened unto the voice of thy wife, and hast eaten of the tree, of which I commanded thee, saying, Thou shalt not eat of it: cursed is the ground for thy sake; in sorrow shalt thou eat of it all the days of thy life;

God did not tell Adam that he hearkened to Satan as had Eve; but instead had Harkened to Eve. Therefore, Adam was not deceived by Satan and was fully aware that he was disobeying God's word. That fact will be of importance when we get to the part about being like Jesus. So now let's take a long look at the part about sin not being levied until the law. Until the arrival of Jesus, God had only given his rules to only two men Adam and Moses. Without rules one cannot be charged with disobeying. So when Paul says that death reigned from Adam to Moseseven unto those who had not sinned as Adam had; All of those people who had not intentionally disobeyed God also suffered death; though they were sinless, as there was no prohibition other than not to eat from the tree of the knowledge of good and evil.

That all changed after God gave Moses the ten Commandments. Now God had decreed eleven rules to follow. Therefore, when one of those rules was broken the person was now committing the same sin as Adam that of intentionally disobeying God. Those who had not intentionally sinned had not lived faultless lives, indeed many had committed acts such as Cain killing Abel, but until the ten Commandments were given there was no rule about killing.

If I have misinterpreted your question please let me know. As far as your last paragraph it is the belief in Salvation that is the basis of Christianity, and the hope of eternal life. Otherwise After death there is nothing, and The soul does not exist, we simply die and are gone.

First a couple of disclaimers: (1) Personally I'm skeptical of motives & specifics of "theistic evolution" as it's called, and (2) though I'm a great admirer of C. S. Lewis I don't agree with or endorse 100% of everything he said or wrote.

That said -- and considering that this site is not about "truth" per se but rather the views of different Christian denominations and traditions -- I offer this quote from C. S. Lewis' book "The Problem of Pain."

In it, he describes his own hypothetical interpretation of creation, which apparently sought to harmonize the evolutionary beliefs as they stood in his time (mid-20th century) with Christian doctrine.

In doing so, he offered an answer (valid or not) to the question you're asking here.

For long centuries, God perfected the animal form which was to become the vehicle of humanity and the image of Himself. he gave it hands whose thumb could be applied to each of the fingers, and jaws and teeth and throat capable of articulation, and a brain sufficiently complex to execute all of the material motions whereby rational thought is incarnated. The creature may have existed in this state for ages before it became man: it may even have been clever enough to make things which a modern archaeologist would accept as proof of its humanity. But it was only an animal because all its physical and psychical processes were directed to purely material and natural ends. Then, in the fullness of time, God caused to descend upon this organism, both on its psychology and physiology, a new kind of consciousness which could say “I” and “me,” which could look upon itself as an object, which knew God, which could make judgments of truth, beauty and goodness, and which was so far above time that it could perceive time flowing past…. We do not know how many of these creatures God made, nor how long they continued in the Paradisal state. But sooner or later they fell. Someone or something whispered that they could become as gods…. They wanted some corner in this universe of which they could say to God, “This is our business, not yours.” But there is no such corner.

If Adam was not literally created by God, and if the Genesis account of creation is allegorical, then everything written in the Bible about sin, death and the need for Christ Jesus to come to earth to die for our sins is a lie.

The Genesis account of creation says that birds were created with sea creatures on day five while land animals were not created until day six. This is in direct opposition to the Darwinian view that birds evolved from land animals. The literalist account says birds preceded land animals. The theistic evolutionist view says exactly the opposite.

Theistic evolution denies that Adam and Eve were created by God in His image, and that because of Adam and Eve's disobedience, sin entered into the world and death through sin.

If Adam and Eve did not exist and they were not our "common ancestor" then there would be no need for Christianity and there would be no solution to sin and death would reign supreme. There would be no hope for sinful humanity.