I really don't think NK is stupid enough to launch nukes, I doubt anyone really is, but I wouldn't put it entirely past them to provoke SK into making the 1st move to continue the war. Remember, technically, they are still at war, as a peace treaty was never signed.
As for China, I doubt they will get physically involved should a war break out. They will protest and make a lot of verbal noise, but will rather sit back and watch the US take hits. This will leave the US weaker financially etc and that is exactly what China would love, as they will then really go on the offensive to stake their claim as the most powerful nation on earth, financially, militarily, population wise and anything else to make themselves feel important.
