You’re right that our knowledge of Adam & Eve is limited by virtue of the fact that we cannot directly relate to their state in Eden, even if we still share a basic nature in common which should help us identify with them to a great degree. But their original giftedness exceeded ours, of course, even after we’re justified at Baptism.
He didn’t start with a fallen nature, as in a corrupted or wounded nature, lost and separated from His Creator. God didn’t create Adam that way. But, while good, Adam wasn’t perfect either apparently. And, as has been mentioned several times, God did create His universe in a “state of journeying to perfection” according to the catechism. So it can be assumed that Adam was given sufficient knowledge to be held responsible/culpable for his act of disobedience, but perhaps not so perfect in wisdom or knowledge that he should be considered irredeemable by God for his act, Who, after all, knew beforehand that humanity would fall. Culpability varies in worldly crimes as well, with different punishments or penalties imposed accordingly. At any rate in this case God is *producing *something in Adam, and in us, that is perfected by trial.