Either the trees were figurative, as in a figurative account, or they were trees that were special, but not necessary. The tree of life is supposed to be in sight in eternity as well, but not necessary to give immortality. The tree named knowledge of good and evil did not impart knowledge. As a counter part, the tree of life did not impart eternal life. While being literal trees, they were not magic trees. They represented a choice, that God would change existence if they were eaten. The tree of life, more than likely only reversed God’s action, after eating of the tree of the knowledge of good and evil.
The lie of Satan was that the forbidden fruit would give them wisdom and knowledge that God was withholding. The result was not what the fruit would accomplish. It was the act of disobedience that caused a result.
Humans were immortal because they were the image of God, not because they had an Immortal body of flesh. The part about entropy was that if there was decay, it was at a slower pace than we have today. Was that because the effect of the sun was different, and not that entropy did not exist?
If we are going with the earth is billions of years old, and God is a being of light, then humans as part light beings could have lived for billions of years as well. We call it spiritual, but if it was a body that the physical did not change, then this spiritual side was something that would keep the flesh constantly regenerating with this other body aspect. But it was not because of some physical aspect of the material universe.
Unless Adam was harvesting this tree and offering to all the other humans, or they all had a tree like it in all their dwellings, this was just a decorative tree, that took up space in the Garden. Adam and Eve were not the only humans. They were either in charge over the others, or at least some spiritual go between God, and the other humans. At some point God took Adam from living with the other humans and placed him in charge of naming the animals and tending to the Garden. Adam and Eve were the major contributers to the current human genetics. There is no way to tell if the spiritual side had a genetic makeup that would alledgedly mix with Adam and Eve’s dna. Other than Genesis indicates there was some genetics that gave some offspring an enhanced physical body.
During the course of a few thousand years, with a new generation every 20 years, Adam and Eve’s offspring would be significate, especially if there was offspring from Cain and other human families introduced into Adam’s genetic dna, even if they had a slightly different dna, because of the spiritual body they existed in. Obviously after the Flood this original dna effected humans thousands of years later.
The situation before the Flood was probably there were more of Adam and Eve’s descendants, than any of the original human’s. The original humans would not need to have as many offspring because death and decay was negligible. The need to reproduce was not there after a few generations. A generation could even represent thousands, and even millions of years if we are going with 4 billion years passing since they were given charge over the earth. They have to be viewed as God living on earth, as in God’s image. They were in part light beings, not just physical flesh.