One thing that I always consider , and mentioned in other places , I even think to one of your college mates, is this.
In the story of Adam and Eve it opens up with them in a garden and only one tree was off limits. The tree of knowledge of good and bad. All other trees were ok, including the tree of life. Now the tree of life gives eternal life.
My first question would be of death did not exist, and Adam and Eve was immortal then why would there be a tree that grants eternal life in their garden? If they could not die, why have that tree there? If they would only need that tree if they were dying because they lost their immortality due to sin, but if they sinned they would be cast out unable to eat from it then it makes the tree really useless.
So how could I create a situation where that tree makes sense.
For me it’s that Adam and Eve were not immortal. They were mortal and so they needed that tree to eat from. That tree sustained their life. I have no reason to believe that one bite from the tree meant eternal life no matter what. I believe that eating regularly for that tree sustained life eternally. It was a gift from God to Adam and Eve in the garden.
That’s also why I believe that Cain was afraid of others beyond Eden. I don’t believe it was his brothers snd sisters but other humans. Humans that were perhaps jealous over Adam and his family being called out as Gods chosen people. That they are safe in the garden and that the others were even afraid to come near Eden. They avoided it. When Cain was cast out , he was afraid of what these others would do that lived well beyond the garden and Eden.
The Bible said that sin has always existed, but before the law was given there was no accountability to it. ( Romans 5:13 ). That law can’t just be to Moses. After all Adam sinned as well. He was given laws and broke it.
So sin did enter because of Adam because Adam was the first human God reached out to and made a covenant with. He was the first man able to break that covenant.
Now because Adam was given a tree that sustains eternal life , when he sinned, and lost it, for him he did begin to die. It does not matter that others could die or that animals could die because the story opens up with them in the garden, blessed with everlasting life. We also have to consider that Adam did not just drop over dead. He from dying until he died. But there was more lost than physical death.he was now also able to be dead in his sins. Something God would have to judge. The first man open up to the judgement of God.
So for me all of this ties I to the questions you asked.