In my understanding, a free will (moral choice), and an eternal soul are fundamentally linked to being made in the image of God.
Based on this, if the humans prior to Adam were also made in the image of God, they would have had the ability to make moral choices, and therefore must have sinned.
The other option (given my understanding) is that they were not (yet) made in the image of God, and therefore were essentially animals, unable to make moral choices.
I think what your are suggesting, is that there is a 3rd option, that their choices would not represent sin, as without the law they had no option to reject God’s law. While I think I understand this in “theory”, I think I’d have to change my understanding of what it means to be human in a relationship with God, and what sin really is. But that’s partly why I’m here, to have my understanding challenged. I guess I’ve got a few more topics to add to my list for indepth Bible Study.