The problem of course is that these books have been written. This tends to support the notion that there may be truth to something within their pages.
The mere fact that a book was written is no guarantee that its pages cintain any truth and no truth wuthin a book guarantees that anything else in a book would also be true. The point though is that right or wrong human beings tend to look for comfort in religions and since we both agree that the bulk of religions are man made that means that in the absence of abrahamic religion another sort would almost certainly take it's place. I don't see that your objections are relevant to the thought experiment.
Yes, I used the word "tend", not the word guarantee. My point in a general sense is that "where there is smoke, there is probably fire". And the fact that discussing contingencies on such a hypothetical basis and intentionally dismissing probably the most well known religions seems to be a question that is based not so much on curiosity as spite.
I don't look for comfort in my religion. Yes, I know you will probably disagree with my assessment. But the other point here is that God is not bound by a book. The OT Jews lived for a long time knowing God prior to the book being written. Books record things - but prior to books - people remembered things. And not like Chinese whispers - but really remembered things. We have lost this art today. But in some countries around the world where books are not utilised very much, these stories of the past are remarkably accurate. Books I think provide for education - reading and writing. The Israelite nation had a 50% literacy rate. All males had to read to pass their religious rites. Well before most nations - probably next to China and possibly Egypt - one of earliest education nations.