Another idea for the betrayal - Peter, having always aligned himself with the biggest people around him (Sirius and James and to some extent Lupin in school, which leads me to believe the disputed fact that he's a Gryffindor - the others are, and they are the biggest in Gryffindor if not in the whole of Hogwarts), recognizes that the Order is losing. Voldemort recruits Peter because Peter is a weak link - loyalty to the biggest bully supercedes his affection and dependence on his friends, though it doesn't necessarily overtake them. Peter succumbs, but the price of loyalty to Voldemort is high - I would imagine that Voldemort demanded a sacrifice of some kind, the greatest kind, and James and Lily equal that. I doubt Peter hated either one - on the contrary, by the time of the betrayal, he's almost deified them both, and sacrificing one's gods is the ultimate test of loyalty to a new god.

Interesting essay. You bring to light some interesting points (esp. Peter and Snape's similarities).