China has influence in NK but they're not directly calling the shots.

NK is playing the same old game as always. Only this time a bit more close to the red line. And they've pretty much backed themselves into a corner. In previous situations they always had an option left. For example: "we'll stop testing nuclear weapons and the US gives us food or takes us off the list of states that sponsor terrorism". Now I don't see something like that.

China wouldn't support an attack by NK. But if in the case of a full out war (hypothetically) the US decides to after the regime and not just take a defensive position, China would most likely intervene I think. A US sphere of influence reaching into their own backyard is not something they could live with. Imagine Chinese destroyers in the Gulf Of Mexico.