Why is the de novo creation of Adam and Eve important?

That makes sense. It seems like you are really trying to understand them, and that is a good thing to do. At this point, I’m conveying what I’m observing. Perhaps at some point you might be able to understand more by directly talking with them (and those with their concerns).

So, Ken grants that this is the case. He can imagine how this is not de novo creation. however, in the case of Eve, it is harder for him to make that leap.

That is a different question, and a good one. I list out several in the book. Here is one reason. Some understandings of original righteousness require de novo creation. Another (as stated by @dga471) is the parallel between Adam and Jesus.

In the cases I’ve presented, it is not about original sin. Rather the concern is about inerrancy.

Yes. We know they have no problem. Both Ken Keathley and Jack Collins endorse the GAE effusively (have you read their endorsements?). They both agree that Scripture does not map to scientific categories like “biological human”.