I am impressed by your effort and enthusiasm regarding the subject of Adam and Eve and “original sin”. To ensure that I have a clear understanding of your thesis, first I will state my impression.
1) You believe human beings are evolved creatures.
2) You believe God endowed Adam and Eve with free will and a sense of law.
3) Adam and Eve then had the capacity to resist selfish instincts derived from evolution.
4) Once they disobeyed God, they reverted to their evolutionary state of selfishness and were in need of Grace and redemption.
5) Prior to the original sin, they were in a state of Grace, which they lost.
6) All humanity shares the propensity to selfishness and original sin is concupiscence (rooted in desire) and the “propensity to sin” is rooted in evolution.
Let me know if this reflects your thesis. If it does, I would make the following response:
1) I believe Adam and Eve are particular people (couple) and God directly “created” them (mechanism unknown).
2) God placed them in a specific place where they were not subjected to the difficulties of natural events and forces.
3) God taught them, and also I agree with you, they possessed the intellect and freedom to understand God and be in communion with Him.
4) God also placed the “tree of life” in that, through the Grace of God, they may access eternal life.
5) God also placed the “tree bearing the fruit of knowledge of good and evil” in the garden and warned them not to partake of its fruit.
6) Eve was tempted by the snake (Satan) and thus succumbing to deceit, was instilled with a desire to “be like god” through the knowledge of good and evil.
The rest is clear from the Bible. Thus Adam and Eve underwent a change in their nature, and this nature is in all of us. Since the origin of sin is from the devil, human nature has the desire to sin, but humanity is not the source of all evil, and thus God has provided a way to repent and overcome the works of the flesh. However evil (separation from God) entered the Creation and thus the need for redemption.
My comment on "why Eve could be tempted and Adam follow this" is because I cannot understand why she would desire "to be like gods" when they could partake of eternal life and be with God. This is a mystery and perhaps has to do with the subtlety of Satan.
The subject matter is vast and I think we should confine these discussions to specifics.