I'm not a teacher.
1. In my opinion you can say "a book-devourer"; you can also say "He devours books", but it should be put into the apprioriate context and said in a figurative way.
2. Could a predator be a synonym for a devourer in any circumstances?
I can imagin the situation where a predator may be a devourer, e.g. a lion, but it doesn't mean the nouns are synonyms. Generally, you can't always subsidise the word devourer with predator simply because not every devourer is a predator.
Student or Learner