I'd add another category: the category where the non-POV character has reason to think there isn't consent, but the reader knows there is.

See, I wouldn't call that dub-con. By definition, if the reader knows there's consent, then there's consent. That could be called rape fantasy, maybe, but not dub-con or non-con. YMMV.

I've seen authors manage to pull this off from both sides at once -- i.e. they *both* think they're taking advantage of their drunk friend (whom they've been secretly lusting after).

Yeah, that was a popular trope in my first slash fandom. It's fun if the writing it good enough to pull it off without making both characters seem stupid.