The doctrine of original sin began as argument that unbaptized babies went to hell, or purgatory. It was one of the main arguments AnaBaptists fought against.
Humans were never immortal and we never had flesh that was immune to sin. That’s why a tree of life ( tree of fruit that sustains life) was in the garden. Immortals would not need that.
When I’m home I can present my thoughts better. Though it’s going to take weeks and be in sections.
Again, I don’t believe in the doctrine of original sin. I don’t think it’s a scriptural argument at all. I don’t really know if you understand my view or not to be honest.