Was God dishonest about the tree of the knowledge of good and evil?

You are correct - the tree of the knowledge of good and evil did not cause Adam and Eve to become mortal and eventually die. However, that is not what God said! God said that if they ate of the tree they would die. The death came about due to willful disobedience of God, the act of eating. The tree was a material object. It can only cause material effects. Moral choices, however, have spiritual consequences. The spiritual death of Adam and Eve was therefore caused by their disobedience. The material death of Adam and Eve was losing contact with the source of life, possibly some toxic material effects from the fruit of the tree of knowledge, which over time led eventually to their physical death.

Some say that God exaggerated that the death would happen on the same day, calling that another lie. However, in Scripture, God sometimes speaks of a prophetic day as being a thousand years. Thus Adam died at age 930 years, almost 1,000 years later and within the same prophetic thousand-year day. This nearly 1,000 year grace period shows up elsewhere. For example, when God decrees in the time of Noah that man's time shall be shortened to 120 years, maximum human longevity did not immediately change. Genesis records steadily decreasing lifespans. The last person to live 120 years is Moses. According to the Biblical chronologies, Moses died within twenty years of the close of the grace period, almost 1,000 years after the decree to shorten the maximum human lifespan to 120 years.

The two trees represented a choice between: living God's way, and receiving eternal life; or deciding for oneself how to behave, and receiving eventual death.

There is nothing dishonest or inconsistent about it.

The same choice was given to the Israelites:

“See, I have set before you today life and good, death and evil, in that I command you today to love the LORD your God, to walk in His ways, and to keep His commandments, His statutes, and His judgments, that you may live and multiply; and the LORD your God will bless you in the land which you go to possess. But if your heart turns away so that you do not hear, and are drawn away, and worship other gods and serve them, I announce to you today that you shall surely perish; you shall not prolong your days in the land which you cross over the Jordan to go in and possess. I call heaven and earth as witnesses today against you, that I have set before you life and death, blessing and cursing; therefore choose life, that both you and your descendants may live; that you may love the LORD your God, that you may obey His voice, and that you may cling to Him, for He is your life and the length of your days; and that you may dwell in the land which the LORD swore to your fathers, to Abraham, Isaac, and Jacob, to give them.”
— Deuteronomy 30:15–20

The same choice is given to Christians today:

“Enter by the narrow gate; for wide is the gate and broad is the way that leads to destruction, and there are many who go in by it. Because narrow is the gate and difficult is the way which leads to life, and there are few who find it.”
— Matthew 7:13,14

“He who believes in the Son has everlasting life; and he who does not believe the Son shall not see life, …
– John 3:36

For he who sows to his flesh will of the flesh reap corruption, but he who sows to the Spirit will of the Spirit reap everlasting life.
— Galatians 6:8

Blessed is the man who endures temptation; for when he has been approved, he will receive the crown of life which the Lord has promised to those who love Him. Let no one say when he is tempted, “I am tempted by God”; for God cannot be tempted by evil, nor does He Himself tempt anyone. But each one is tempted when he is drawn away by his own desires and enticed. Then, when desire has conceived, it gives birth to sin; and sin, when it is full-grown, brings forth death.
— James 1:12–15

Blessed are those who do His commandments, that they may have the right to the tree of life, and may enter through the gates into the city.
— Revelation 22:14

Since we already know that God wouldn't lie (rather, the Devil is a liar, John 8:44), it seems more reasonable to hold that the chain of reasoning you lay out is mistaken in some sense.

Note that Eve at Genesis 3:13 says

"“The serpent deceived me,” she replied, “and I ate.”"

If this is an accurate translation, the serpent did lie when he said at Genesis 3:4

"“You will not surely die,”"

A more direct problem is God says 'on that day you shall die', but Adam and Even don't physically die immediately. One solution to this is to say the death God is directly referring to is spiritual death. This coheres with use of 'death' in other parts in the Bible.

Model 1 for making sense of what's going on is

1. Adam and Eve are created with spiritual 'life' and physical immortality.
2. Eating of the Tree of Knowledge of Good and Evil causes immediate spiritual death, and shifts them to mortal, so physically they will die.
3. But, they might be able to eat from the Tree of Life, which although it will not restore spiritual life, will confer physical immortality.
4. This would create a kind of 'vampire' scenario - physically immortal but spiritually dead.
5. This is not a good thing, so God kicks them out of the Garden and guards the entrance so they will, indeed, eventually physically die.

Model 2 is

1. Adam and Eve are created with spiritual 'life' and on-going eating from the Tree of Life confers what is effectively physical immortality.
2. Eating of the Tree of Knowledge of Good and Evil causes immediate spiritual death.
3. But, they might be able to continue to eat from the Tree of Life, which although it will not restore spiritual life, will confer what is effectively physical immortality.
4. This would create a kind of 'vampire' scenario - physically immortal but spiritually dead.
5. This is not a good thing, so God kicks them out of the Garden and guards the entrance so they will, indeed, eventually physically die.

This model is motivated by Revelation 22:3, where fruit from the Tree of Life is apparently eaten repeatedly.

Yet, Genesis 3:22 suggests Adam just needs to eat once from the Tree of Life to achieve immortality.

"And now, lest he reach out his hand and take also from the tree of life, and eat, and live forever..."

If that's the case, we can have

Model 3 is

1. Adam and Eve are created with spiritual 'life', and are intended to eat from the Tree of Life and so also have physical immortality.
2. Eating of the Tree of Knowledge of Good and Evil causes immediate spiritual death.
3. But, they might be able to eat from the Tree of Life, which although it will not restore spiritual life, will confer physical immortality.
4. This would create a kind of 'vampire' scenario - physically immortal but spiritually dead.
5. This is not a good thing, so God kicks them out of the Garden and guards the entrance so they will, indeed, eventually physically die.

Q. What reason do we have to think Adam was immortal?

A. In the NT, we have 1 Corinthians 15:21

"For since death came through a man"

and Romans 5:12

"just as sin entered the world through one man, and death through sin"

If death came through Adam's sin, it sounds like Adam was physically immortal before sinning.

However, as the alternate models show, it could be that physical death came because Adam couldn't be allowed by God to continue eating or eat once from the Tree of Life once he entered into spiritual 'death', or this would create very large problems.

Introduction

The key to understanding events related to the tree of knowledge of good and evil is to understand that God was not speaking about physical death, but about spiritual death.

If God was speaking about physical death, then God was indeed a liar. In Genesis 2:17, God is quite emphatic that if the humans (Hebrew: אָדָם) eat from the tree of knowledge of good and evil, they will certainly die, and they will die on that day:

and of the tree of knowledge of good and evil, thou dost not eat of it, for in the day of thine eating of it—dying thou dost die. (Genesis 2:17, Young's Literal Translation, emphasis added)

The certainty of death

"Dying thou dost die" is a literal translation of a Hebrew idiom that gives strong emphasis to a statement. Hence the King James Version translation:

But of the tree of the knowledge of good and evil, thou shalt not eat of it: for in the day that thou eatest thereof thou shalt surely die. (emphasis added)

And the New International Version translation:

but you must not eat from the tree of the knowledge of good and evil, for when you eat from it you will certainly die. (emphasis added)

Translations such as the Revised Standard Version and the New Revised Standard Version that do not include some indication of strong emphasis are not properly translating the original Hebrew.

The immediacy of death

Further, the Hebrew is quite clear that this death will occur on the day that the humans eat from the tree of knowledge of good and evil.

Some translations, such as the New International Version, soften this by translating the Hebrew as "when you eat from it, you will die." But the original Hebrew clearly says "on the day you eat of it, you will die."

Yes, it is possible to argue that these are 1,000 year "days." But that is an unnatural reading of the Hebrew here and elsewhere where this stratagem is employed in an attempt to avoid the obvious meaning of the Bible text.

The obvious meaning of the verse is that on the day that the humans eat from the tree of knowledge of good and evil, they will certainly die.

Who lied, God or the serpent?

God said that on the day that the humans ate from the tree of knowledge of good and evil, they would die.

The serpent disagreed:

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." (Genesis 3:4–5)

If God was referring to physical death in Genesis 2:17, then God was a liar, and the serpent was telling the truth: on the day that they ate from it, the humans did not die physically.

And yet, the Bible assures us that the serpent, or the Devil, is the liar, whereas God tells the truth. (I hope I do not have to cite the verses for you.)

God was talking about spiritual death

The only reasonable conclusion is that God was not talking about physical death. And if God was not talking about physical death, then the only conclusion left is that God was talking about spiritual death.

This is precisely how Emanuel Swedenborg (1688–1772) interprets Genesis 2:17:

Genesis 2:17. "But from the tree of the knowledge of good and evil you may not eat, because on the day on which you eat from it you will surely die."

The meaning of verses 16–17 is that people are allowed to depend on any perception they receive from the Lord to tell them what is true or good but not on themselves or the world around them (in other words, when inquiring into religious mysteries, they are not to rely on sense impressions or facts); and that to do so would be the death of their heavenly quality. (Secrets of Heaven #126, emphasis added)

For Swedenborg's full explanation of this verse, read the subsequent sections, from 127 to 130. Here is the short version:

• The tree of life represents relying upon the Lord to show us spiritual truth from within, and in later times, turning to the Bible as our primary source of knowledge about spiritual things.

• The tree of knowledge of good and evil represents turning outward, to our physical senses, as our primary source of knowledge and reasoning about spiritual things.

The latter will lead to denial of everything spiritual, and thereby to our spiritual death as we become materialistic in our attitudes and in our life, simultaneously turning our back on the Lord and the Bible.

The former will lead to acceptance of everything spiritual, and to spiritual life, as we turn to the Lord and the Bible to learn about God and spirit, simultaneously putting them into practice in our life.

God told the truth; the serpent lied

If we understand that in Genesis 2:17 God was talking, not about physical death, but about spiritual death, then we can see that God told the truth, whereas the serpent lied.

On the day that the humans (now known as Adam and Eve) ate from the tree of knowledge of good and evil, they did die, but it was a spiritual death, not a physical death:

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. (Genesis 3:6–7, emphasis added)

Here is Swedenborg's explanation of the meaning of their recognition of their nakedness:

They realized that they were naked means that they recognized and admitted that they were living in evil and not in the innocence they had had before. This can be deduced from the last verse of the previous chapter, where it says, "And they were both naked, the human and his wife, and did not blush." Their lack of embarrassment over their nakedness symbolizes their innocence, as may be seen there [§§163–165].

The symbolism is just the opposite when they do show embarrassment, as is the case here, where they sewed fig leaves together and hid themselves. When innocence is lacking in us, nakedness is a matter of shame and scandal because we are conscious of wrong thoughts. (Secrets of Heaven #213, link added)

For Swedenborg's full explanation of Genesis 3:7, see Secrets of Heaven #211–217.

In short, the death that the humans experienced on the day that they ate from the tree of knowledge of good and evil was the death of their innocence.

The death of our innocence is a spiritual death.

When we are innocent, it means that we have done no wrong. It also means that like sheep, we are willing to be led by the Lord.

When we lose our innocence, it means that we have done wrong. It also means that we are not allowing ourselves to be led by the Lord, so that we are sheep who have gone astray (see Psalm 119:176; Isaiah 53:6; Matthew 18:12).

The serpent lied. He denied that the humans would die on the day they ate from the tree of knowledge of good and evil, when he knew full well that the humans would experience the death of their innocence, and along with it, the death of their spiritual life in harmony with the Lord.

The serpent also lied in directing the humans' attention away from the spiritual truth that they received from the Lord from within, toward a physical-minded and materialistic view of life. If the humans believed the serpent, they would believe they had not died on the day they ate from the forbidden tree, because they did not die physically. So the serpent induced misunderstanding and falsity on the minds of the humans based on an external, physical-minded understanding of God's words, when God's words were meant to be understood spiritually.

Conclusion

God did not lie when he told the humans that they would certainly die on the day that they ate from the tree of knowledge of good and evil. No, God told the truth, and it was the spiritual truth. On the very day that the humans ate from the tree of knowledge of good and evil, they would die spiritually. That spiritual death was the death of their innocence. This is exactly what happened on the day they ate from that tree.

In short, God told the truth, and the serpent lied.

And he shewed me a pure river of water of life, clear as crystal, proceeding out of the throne of God and of the Lamb. In the midst of the street of it, and on either side of the river, was there the tree of life, which bare twelve manner of fruits, and yielded her fruit every month: and the leaves of the tree were for the healing of the nations. - Revelation 22:1-2

I think the OP contains an assumption that gaining eternal life from the tree of life is a "once and done" kind of affair. No such stipulation is voiced in Genesis for the tree of life. Revelation seems to indicate there there is a continual, year-round production of the life-giving fruit in the New Jerusalem. If eating from the tree once is enough to provide eternal life and the number of people in New Jerusalem is finite, why the continual fruit production?

There is such a stipulation stated in Genesis regarding the tree of knowledge: In the day you eat of it you will surely die. This is a once and done affair. And so it happened. In the day they ate of it access to the tree of life was barred from them and their progeny and death came upon all.

There is no lie and no inconsistency.