A: I wouldn't bother taking offense. It's really not worth it, and I don't think he meant badly by it. He was only stating a thought he had based on observations, and that thought was not far fetched. It was a simple misunderstanding. Don't sweat the small stuff.
A: The fact that you even thought about this question should be good enough to prove to her that you love her deeply. Hell, even the fact that you're asking her to marry you is a good indicator.
A: Well, actually, guys may be slow, but they're not stupid. You sent a pretty clear "don't want to talk to you" vibe. So I think you'll have to open contact again.
A: I'm sure that even if you wrote him a note, he'd accept. That sounds right, given what you've said. If he doesn't ask you out. Just be sure he gets the note, cause if he doesn't, a bad message will get sent in both directions, if that makes sense.