China don't really give a damn about North Korea because they want that buffer from the United States. In a sense, the country is a pawn in this 'new Cold War'. You can bet your bottom dollar China will intervene and try to defuse the situation quickly on the basis of pre-existing tensions. The reason they haven't been so vocal in the past few weeks is that you'd look absolutely mad trying to defend some the calls made by NK.
In the situation that NK gets destroyed, I'd be very curious whether reunification would even be on the cards. Of course you've got the cost of rebuilding infrastructure for at least half the peninsula along with the massive shock of reintegration. Also, China won't sanction a move unless United States retreats from SK. In the end, this becomes a situation which mirrors the Cold War and relations between the US and China will be further strained.
You can definitely rule out any peace pact with the US. Koreans are really into ethnic nationalism and anything which "betrays the race" will be a national humiliation. Even in SK, there's more anti-Americanism than one might think. It's a sad state of affairs but I don't really see a real change in the situation. The best it COULD get is NK moulding itself based on China's economic and governmental structure. I don't like those odds though.