Wow, that was surprising to read as an American. Here, "cunt" is universally a negative + derogatory term, without exception.
Too much familiarity with someone you don't known is a highly aggressive challenge. "What's up motherfucker" to some guy at the bar won't make any friends and might end in a fight.
No it isn't. It's often hilarious, as in the above example. And in that one episode of Curb Your Enthusiasm. You wouldn't use it in mixed company, but if you know your audience you can say any number of horribly offensive things.
Another example would be if I told a friend something incredulous, they might respond with "get tae fuck!" (meaning, "fuck off!"), or "awa' an shite!", (meaning "what a load of shit!").
Because there would be violence involved.
Seriously though, the tone, context and body language would be completely different.
I'd rather my friends call me a cunt (as the closest ones do) than mate, because the thin veneer of politeness in mate implies distance if not contempt (from strangers / acquaintances).
It still has negative connotations, but in a much lesser way so it becomes part of friendly banter.