I really think that the humanitarian problem is not a real problem for China. A country this size could easily swallow 10 million Koreans (let's be real, surely not more than half of population would flee), allow them to work and assimilate and not even notice it really. I think the issue is that China is afraid that united Korea will become a US ally and allow US forces to station there. Also it could expose China to the world as a weak player that could not handle Korea by itself.