Yeah, I think Rowling went a bit far in the redemptiveness of Snapes actions. He did the right thing, but really it was for the wrong reasons, IMO. Harry naming one of his sons after him, even if it is only a middle name, is a bit much. Whatever he did to help defeat Voldemort, Snape was still a bigot who delighted in bullying eleven year-olds.

Harry is never seriously tempted by the dark side, true. He's plagued by doubt and tempted to stray from the path set out for him by Dumbledore, to claim the Hallows for himself, to pursue power. Harry never went dark and returned, but he was tempted and succeeded in resisting temptation. It's true that Harry doesn't change dramatically, but he grows as a character, learning to deal with responsibility and to trust others.