No, see, they want NK for arms length plausible denial when they want to do something that cannot be defended in public, without taking the heat for it or compromising relations (especially trade) with the west. If they want to send missile or nuclear tech to somebody, they just give a nod and pass it through NK. If they want to kidnap people or blow up airliners, the NKs do it, the Chinese say they are against it but do nothing to rein in NK.
If they want to lob missile over Japan, or hold people up for monetary tribute, they let their proxy do it. If they ever want to fire a nuke at the US, or smuggle one here, or sell one to Islamic terrorists, they won't do it direct and risk a return address of "Peking" - they will let NK do it, providing whatever tech, support, money, diplomatic cover etc is actually required.
NK is simply a criminal front organization for China's illegitimate state interests. It works perfectly, too. They are sending NK technical materials right now to allow NK to extract plutonium from reactor fuel, thus directly furthering nuke proliferation. But nobody calls them on it, because they say one thing and do another. NK might get blamed - allow appeasement runs rampant even there - but nobody would dream of consequences for China for the present nuke flap. Why, they are so indispensable, after all. Can't possibly afford to upset them.
It is the simplest sort of indirection and dodge. The amazing thing is simply that there are people gullible enough to fall for it. Or bought enough. Or who prefer a Chicom win in the long run.
This is sort of true, but its more than that. What happens if the rest of Asia turns on N. Korea? Or N. Korea falls? China is thinking much longer terms.
They do want to do what you are talking about in the above sentence, but do it politically also. Not just militarily or otherwise.
War cannot be sustained forever, but political balance is what they want.