But things are going to be terribly uncertain in the future for the DPRK. We will have to work extremely hard to restabilize the new, free North
You need to be able to eat to rebel. North Korea never really got over the 90's famine. Food-wise, conditions there are slightly better there in that they don't need mass graves to get rid of all the famine corpses anymore, but that isn't much of an improvement. Ironically, their instability has kept them stable and will continue to do so once the DPRK falls.
In any case, once the DPRK falls there won't be a North Korea anymore. It is beyond question that South Korea will reabsorb them once their government falls.
China's going to want to keep some sort of North Korea around, preferably a communist North Korea. They want it to be stable, but they like having an Ally that close to keep South Korea occupied. They won't want a unified korea unless it's a communist Korea, whereas the U.S is the exact opposite.
Neither China nor North Korea are communist governments any longer. NK removed all references to communism in their constitution a while ago. Now it's just god-king rhetoric for the Kims. China officially supports "Socialism with Chinese characteristics" and a "Socialist market economy". If you ask the Communist Party of China they'll claim they're still Marxists, but nobody buys that anymore.
As I've said before, China may not be thrilled at the idea of a unified Korea dominated by South Korea, but North Korea isn't good for them either. They're fucking crazy and make China look bad in a period of time where China wants to at least appear squeaky clean in order to expand their power base to match the US. At the end of the day, there is no great way for this to end for China, but I think they'd be willing to at least accept the unified Korea as long as the involved diplomats are skilled.