I really wished they would have used it in OoTP per the book instead of Harry breaking through Snapes defenses.
I'm almost certain that's how it did happen in the book....Harry witnessed Snape's memory during an occlemancy lesson.
You are right. But it isn't till end of the series that he understands why Snape acts the way he does towards Harry. (Trying not to give anything away to those who haven't read Deathly Hollows.)
Actually that isn't correct. If you remember, Snape would remove memories before his lessons and place them in a Pensieve. During one lesson, Snape left the room for some reason and Harry delved into his memory through the Pensieve.
For the movie delay, did they really give a reason besides Potter movies do well in the summer and some vague reference to the actors strike?