I just interpreted the lack of Harry thinking of Sirius and Hermione perusing the library as Rowling wanting to keep the book more focused. I'm sure that they occurred frequently during the half a year plus worth of days that she didn't specifically touch on for the duration of the story. Same with Snape's defense against the dark arts lessons.

I did see the lack of Hagrid as a little disturbing. Perhaps Warner Brothers was putting some pressure on her? I seem to recall a rumor that Robbie Coltrane's contract only lasts through movie five.