I wonder, because it seems to me that the people in charge must understand North Korea's place in the world. They must know that the party is over the moment that nuke hits a civilian population. I don't think they'll do anything more than posture themselves and bark loudly. If they were that ignorant to their own situation they would have created some real conflict by now, but they haven't, and they won't.
It depends what they have, eg. if they managed to successfully construct a Hydrogen Bomb, then considering what an unstable maniac Kim Jong-un is, who can say if he wouldn't use it.
He has threatened to fire missiles at Guam this week, I think it's on Thursday, also there's a new rumour they are going to attempt to test a new missile, of course their track record of successful missile tests is pathetic as they always fail.
I know this doesn't answer your OP questions, but I'm not sure they can be answered, it's impossible to comprehend the bizarro thinking pattern of Kim Jong-un.