It comes down to who reaches Pyongyang first. China can't get infantry to the DMZ within a day obviously, and the South Koreans will be crossing in force with their American allies. The circumstances of the outbreak of war, and who occupies what by the cessation of Northern resistance, will determine the terms of reunification or joint governance.The South, having suffered so much would at the very least want to push the DMZ 100 miles further North to protect themselves - and as before why stop there? Would China just hope that they establish a smaller North Korea under UN oversight or would they preemptively secure this themselves just as the Russians did?
China can't really hope to grab the whole thing for itself, which is why kicking the can continues to be the best option. The best case for China is Chinese-militarized Changbai Mts, and US troops leaving the peninsula within 5/10 years.