Referring to Adam and Eve you stated,”I don’t think she or her husband or her children would have sinned if they’d been left alone.” Were not the heavenly creatures created without a sin nature? Wasn’t it their free will, which is the ultimate issue, that caused them to rebel in spite of a near-perfect environment? After all, sin is rebellion against God. It seems to me that the single, most important lesson of the Bible is that without surrender to God’s perfect will ALL intelligent, created beings will eventually rebel (sin).
How long have the angels been around? According to most scholars 2/3 of them have resisted the tendency to sin, and had Satan not led the others astray who’s to say they couldn’t have resisted too. Eve was also deceived into sinning by Satan just like the fallen angels before her. Adam made the choice to follow Eve rather than lose her. This caused all of their descendants to have been born with a sin nature, unable to keep themselves from sinning. In all of creation, Only Satan made the choice to sin totally on his own, free from either external influence, or an inherited deficiency. Take Satan’s influence out of the equation and it’s impossible to conclude from Scripture that someone would have eventually sinned.