Korean immigrants are no big deal to China, its not as if they will be burdening the Chinese welfare system is it?
China is very happy to have North Korea between them and any American bases. In short, North Korea is not a really big problem for China.
America cannot destry North Korea by military means no matter how much hot air is expended on the topic. The DPRK is dug in like no other country has ever been and they have (a few) very sharp teeth. Besides, the last decades should have shown the error of believing that a few determined people cannot do unacceptable damage to a first world country. Forget all macho fantasies about 'shock and awe' with respect to North Korea.
The DPRK is at war with the ROK and the USA, they are only in a state of truce. Of course bringing that to a close would change the situation somewhat. Meanwhile, holding live firing exercises in a disputed area must put the truce at risk and can be only considered provocative.
Meaningful engagement with Pyongyang is the the only reasonable way forward but it will take a long time to recover ground lost since the Agreed Framework of 1994.