QUESTION #1:
If there were no Holy Book in existence--what would you or anybody else KNOW about God and how would you know it?
If we had a decent scientific understanding at the time, we would know nothing of him — It's like if cars were invented today — gas would not even be an option.
No fear of the unknown = No gods.
QUESTION #2:
If whatever Holy Book(s) turned out to be unreliable for any reason (or at least hit-and-miss) would that be any different from being totally wrong?
No. This is why I encourage the believers to get surgery from a surgeon that operates with the Biblical knowledge of 2,000 years ago. After all, they trust their lives with the book.