Oh, I agree with you. The movies are like authorized fanon in a way. There is so much that doesn't resemble canon it's ridic. BUT one thing is I think they have ALWAYS made Snape less of a bad guy IMO and I can totally believe they would play his love for Lily sympathetically. So I have my fingers crossed that that scene will show what I see when I read that passage from the book, which is the two of them having one moment to see each other more clearly than they ever have in the past.
HOWEVER, I also know that the more important *I* think something is, the more likely they are to fuck it up. :P
So overall I am actually dreading the final films but if they 'soften' Snape up enough I think it could be great for us.