that doesn't even make any sense. you are suggesting then that the USMC invade myanmar so that they can invade china? through the thick jungles? by invading myanmar they would have already tipped off the chinese, the myanmar military might be a pushover for the US but not so quick that the USMC can take it over without anyone else in the world knowing. oh and the US just blatantly invaded a neutral country in an act of naked aggression here.
and to be clear i dont think the nuke will be involved. for one this war is extremely unlikely, and secondly, theres not a chance in hell the US can hope to occupy china, in fact, its unlikely the USMC can do much against the PLA at all, if the US were to pull all of its forces from elsewhere and involve all branches of its military it might be able to take some coastal cities(with appalling casualties on all sides) , even then probably not for long, the PLA is simply too strong on its own homeland.
and given there is no nuclear guaranteed between china and pakistan im pretty sure i can say china is not going to fight a nuclear war for pakistan, however, your point that the US arsenal is much larger mean exactly nothing. blowing up the world 50 times overs isnt more threatening than blowing it up once over. china only has the arsenal it has because it chooses it, it has calculated that what it has is all that's needed, if tension rise and a real prospect of war appears(wars dont happen in a vacuum, there's always a buildup/tension prior) you really think china doesn't have the tech or weapon to match the US arsenal if thats whats called for?