Maybe the Chinese are just fortifying their frontier in case of massive counter attack by SK. Or if it looks like the Norks are going to be totally overwhelmed, the Chinese will come in and secure NK for their buffer zone.
I think that if push came to shove, they’d rather administer NK than see a unified ROK Korea on their border. I’d be more than surprised to see the ROKs cross the DMZ with conventional forces. That would mean coordination with us, and the approval of the Obama administration, which isn’t going to happen.
China might be looking ahead to the possibility of a President Trump though, with less of a certainty that the DPRK’s usual martial shenanigans would be ignored. They might look at him as a potential President MacArthur, in terms of policy direction.
If they were going to do that, they’d have to do it soon, so as to have it be a militarily-settled issue by the time the next US president takes office. I guess the place to watch is going to be the Sino-Korean border.
It also occurs to me that the ROK might not really be keen for unification. Remembering German reunification- I recall that it put a significant load on the FRG to bring the country as a whole up to speed and I’ve heard that the burden of integrating the DDR is still being felt. If the integration of the DDR was hard, doing so with the DPRK with it’s long-standing isolation would be much worse.