Why can't we put "library" right before the verb and not in the beginning?
Technically you can but it doesn't sound as natural when speaking.
What is 권만이?
권 is the counter for books, 만 is only, 이 is the subject particle
Why is there a subject particle after 권만, should 만 simply replace other possible particles?
"책 예순한 권만이 도서관에 있습니다" wasn't accepted
Why is "있어요" not accepted as the last word?
It is correct just a bit less formal
This might sound stupid, but in this case why is it not 책이/은 ? Is it because the 권만이 already states the subject?
I've noticed that all the counters have the participles instead of the noun
The subject particle goes either on the noun or its counter, which occurs last. Typical order is Noun+Number+Counter+Particle.
Why is 도서권에서 wrong? Since you are indicating that the books are just existing at the library, couldn't you use it like that?
Why cant it be 예순한만?