If they use a nuke, it'll be on Seoul and we all know it.
I doubt it. Seoul is already targeted with enormous amounts of conventional artillery that would level the city in a second.
Plus NK still probably thinks it could conquer the South and re-incorporate it to "unify" Korea, and nuclear fallout would make that more difficult for an invading force. And they still hold a grudge against Japan for previous wars and invasions (but oddly, never a grudge against China for its invasions and meddling of Korea in the past, fancy that...) And Japan's the country they keep launching missiles over and just this week said should be sunk into the ocean.
I'm really worried for Japan. I have no idea how they're not freaking out about it right now. If i were in Japan, I'd be demanding the gov't do a massive military build-up immediately. Both to try and shoot down missiles (ideally before they even get near Japan) and to launch a devastating counter-attack if NK did attack. U.S. isn't going to do shit to help unless a base of ours in Japan is directly attacked, unfortunately.
The U.S. would have to intervene if Japan is attacked. We would have no choice because of treaties. If Trump refuses to help, then we will look weak as a nation. ---