I’m not sure that China “owns” North Korea, or can control the behaviour of the Kim regime. Yes, they can shut down fuel and food supply to an extent, but that would only make for more suffering for the ordinary people and make the elite more desperate and accident prone.
You have a point that the Europeans and Russians would probably not agree to any sanctions against China, which would have been futile in any case, nor will they sanction any military action against North Korea before all other options have been exhausted, or an actual attack on South Korea take place. Sable rattling and war of words by both sides will not change that.
You are also right that if US should impose unilateral sanctions against China, it would be totally ineffective. For one thing there is little if anything US produce that China really needs, cannot obtain somewhere else, or produce themselves.
The other way around is a different matter. Where would US get it’s refined Rear Earth minerals from, needed to produce the electronics it needs to keep it’s military supplied with the latest and best of weapon systems. (Never mind sneakers etc.)
Your point about the Chinese holding trillions of dollars in US debt is also well worth considering.
CNN has a time line about what actually transpired to create the two Korea’s and some facts about the Korean War, which is interesting to refresh memories: http://edition.cnn.com/2013/06/28/world/asia/korean-war-fast-facts/index.html
It is worth remembering that nothing was really accomplished by the Korean War and the nearly 2 Mill. people killed by the fighting and starvation. It ended with an Armistice Agreement not a Pace Treaty, on 27. July, 1953, with the dividing line at exactly the same place as before the war: http://totallyhistory.com/division-of-korea/
That is still the case and any renewed fighting will not change that, only formal discussions involving all involved parties and the UN and without preconditions can solve this problem. Veiled threats by either side is not going to get anywhere, except maybe to WAR!!!