If China were smart they have a great trading partner in Taiwan and in South Korea. If they let NK collapse, a united Korea would not have any foreign forces (except maybe a US naval base for old times sake) and they would gain a pretty peaceful neighbor. As a matter of fact I think Korea would agree in a heartbeat to put no military forces north of the 38 once united.
Agree on Korea, they are like the divided Germany. They would agree in a heartbeat and have actively pushed for reunification. The only block to that has been the north's rigid Stalinist dictatorship witch won't cooperate with the south. I suspect that the north's military government is wary of letting their population (serfs/slaves) see the south's booming economy.
Like the reunification of Germany, South Korea doesn't see the problems of absorbing their northern neighbor with no assets to speak off except a lot of obsolete artillery and a starving population. The North's infrastructure is largely rusting scrap metal. I doubt the south has any desire to become a nuclear power and would probably ask the Chinese for help in decommissioning the weapons program. They might move toward pressurized light water reactors to beef up their electrical grid.
As for Taiwan they also have a "power house" economy and would be happy to trade with most anyone. However, they are wary of the mainland Chinese as they have in the past used Taiwanese real estate for target practice. The mainland Chinese also are rigid in their demands that the "rebellious province" forswear their sovereignty and reunite with mainland China. That point is the deal breaker and I don't see any possible compromise on either side (think Palestine vs Israel). If the communists were to relax their demand for reunification, everyone would breath easier and trade (spice?) would flow.
Regards,
GtG
PS If China were to relax it's demands for the return of Taiwan there is the possibility that trade between the two might increase to the point that de facto reunification happens. They already share a common language and 6000 years of history. In politics never say "Never".
G