The funny thing for me is that, in my mind, China would have more influence and control over a Seoul-led and united Peninsula than they do now. Why? SK will have to build a modern industrial economy from nothing in the North, and China stands to gain quite a few puppet-strings by providing cash, materials, equipment, and semi-skilled labor in a process that dwarfs that of the German reunification that is still ongoing after two decades. Not least of which is that, as ENT said, China could make a withdrawal of US forces in the region part of their requirements for allowing it to happen in the first place... which would negate the need for a buffer state entirely.