If China does support NK in such a situation, that support would be limited to things which are entirely deniable- material and perhaps some light cyberwarfare. Certainly no extra ICBMs, and nothing that would stop the U.S. from rolling over NK in short order (for all that it certainly increase the cost for the U.S.)
It would be extremely disruptive to the region which would result in opportunities to use the Chinese army to make gains on disputed territories while the world was occupied with the big war elsewhere. I did not say this was likely.
In all scenarios I see, it will work the other way around. I doubt that NK military would move a centimetre without Beijing's explicit authorisation.
In any big war, China will use NK as a cannon fodder.
Chauschesku was bas a bastard, bit it was their bastard, unlike Enver Hoxha.
To all outside observers, this looks like North Korea did it. But Beijing is now turned to glass, and the Chinese command is transferred to the military division in Chengdu. The world is tense, as it awaits China’s retaliation.
However, China doesn’t fall for the bait. They begin their investigations in earnest, and discover the true mastermind behind the nuclear attack: The United States.
World War 3 begins. China launches 1000 ICBM missiles at the United States. Thus ending all known life as we know it on American soil. The American military counterattacks and launches all their nukes at China. Both sides are turned to glass.
The world gasps at what just happened. Russia now proceeds to take over the world. Well played.