While China may indeed decapitate NorK, I truly doubt that they will stay there any longer than they must, after setting up a puppet government.
The biggest reason is that the Chinese already have a huge problem with North Korean illegal aliens, and certainly do not want millions of NorK refugees flooding into China. If anything, once they set up the puppet, they will start exporting NorK illegals back to NorK like there is no tomorrow.
Oddly enough, SKor doesn’t want all those illegal aliens, either, or to have to pay to fix NorK, so they are likely trying to assure the Chinese as much as possible that they will not make a fuss if China does something.
The US wildcard is the one thing President Trump could offer to help the Chinese stabilize the situation: a ballistic missile shield in case NorK starts throwing missiles at the Chinese.
Do the Russians have the capability to provide missile shielding with the guided missile cruisers?
News yesterday was there were 2 of them “visiting” SK.