So the only bit of this that doesn't scan for me is, if he does feel this way, then why didn't he ask her to the Ball?

I think he probably didn't even know he felt this way about her until he saw her. I mean, when he's treating her like she's not a girl at all, I think he means it. It's not just that Hermione isn't giving him the go-ahead, it's that he's probably not attracted to her either.

I think you're right not to let him off the hook, actually--a lot of the way I think he's feeling is just lazy. If he'd asked a girl to the ball and she'd gone with him, he might still have felt unnerved by Hermione suddenly being somebody he might have wanted to go with. It's like...to me it seems like before the ball it's not an issue. Seeing Hermione in a dress is suddenly a threat that he could lose her, and that makes him sort of revise all of their previous relationship in his mind. Suddenly instead of Hermione just being herself and him being himself, he probably sees her as witholding something from him, if that makes sense. The irony is that, as you said, he *didn't* ask her to the ball. Had he asked she would have gone with him. But he didn't really want to go with the Hermione he knew.