Some of your questions may be answered if you consider the following:
The story of the man and the woman in the garden is a myth. It is an imaginative story that uses symbols to explore a realty beyond our comprehension. The reality being explored is “Why do human beings suffer?” In its present position, coming after a story nowhere near as old, the question becomes even more mysterious. Given the beliefs that God is all loving and all powerful, that God made humans human beings in God’s own image, and that we are very good, why do we suffer? You would think God would have created things in an order that didn’t involve suffering. Why pain in childbirth? Why death? Suffering appears to be part of the order of things. All of this is very mysterious. How can our belief that God is all loving and all powerful be made compatible with our experience of suffering?
The author who cose to explore this question does not have the option of giving an historical explanation. He doesn’t know a historical explanation. The author makes it very evident that his genre is not historical by his obvious use of symbols.
What in the story is an obvious symbol? Every reader, once he or she thinks about it, recognizes the tree of a knowledge of good and evil as a symbol. Such a tree does not in fact exist in the order of reality. Notice there is no apple tree in this story. There is a tree of a knowledge of good and evil and a tree of life - another obvious symbol. If one can eat every day from the tree of life, one will not die. A third obvious symbol is the talking snake. Notice too that the snake is not referred to as the devil. The snake is a character in the plot, just as God, the man, and the woman are characters in the plot.
You may ask “How do you know that these are symbols? Maybe back at the beginning of creation there were trees like that and snakes could talk.” The story does not date back to the beginning of time. The author is not contemporary whith the dawn of creation. This is a very sophisticated story. At the dawn of civilization society did not have a highly sophisticated view of marriage as expressed in Genesis 2:24. Neither farming nor the establishment of towns was an early development in prehistoric life, yet the fourth chapter of Genesis reports that Cain, who tilled the soil, married and built a town, all while separated from the family of his birth. This story, like the story in which God creates the world in a workweek, reflects a much more highly sophisticated society than would a story about the actual first human beings on the face of the earth. However, when we understand the literary form of the story, questions that presume historicity appear irrelevant. The text will simply not support a claim of historicity.
In short, we suffer when we deviate from the spiritual order created by God.