China has other motives and probably wouldn't risk such a thing even though you are probably right. I'm thinking more like Azerbaijan which I know only they have a border with Russia. I suspect the government like most I know little about is some form of dictatorship and so might be insane. But again my knowledge of the country is near zero and so don't read the above as an accusation only a possibility

Probably not China, but once all the fuel supplies are depleted and the tanks can't move, I could see Mongolia sweeping in on horseback to reclaim their historic lands.