Re: Chinese control
If one looks at the history of the region, Chinese interest has been in securing the border with a buffer. In some respects this is no different to the Soviet approach to eastern Europe.
While the North started the fight, and took it up to the then deployed forces pushing them south to the Pusan peninsula, a counter-offensive cut-off the North forces in the south and headed to the Yalu River. At this point China felt directly threatened and given Gen. MacArthur behavior at the time requesting nuclear weapons, may have had some cause to be concerned.
The Chinese crossed at Yalu en-mass and then this turned into a war of attrition, the Chinese objective met, the land buffer from the west forces established its the status quo they sought.
Prediction is if the glorious-leader/nut job does indeed target the US or US interests, expect China to invade and take control rapidly, replacing the regime with a more friendly puppet, just as they have in Tibet. The last thing China wants/will allow is reunification as it wants the buffer, having western forces on a land border is unacceptable to China.
US wont nuke the North, the fallout would impact key allies, and ultimately the US itself.