It is a possibility that, if they knew N.K. were about to do something really foolish, the Chinese themselves would nip in there an take the 'leadership' off to a nice holiday spa. It would be entirely in China's self interest to avoid a serious conflict so close to its border.
Are you sure about that? China could easily be looking forward to an excuse to deal with the US while it has a weak and ineffective leader.
It is equally likely that China would use an excuse of serious conflict on its border to say "This nonsense is on our border. We're taking over the entirety of Korea and unifying it as a Chinese protectorate."
Are YOU sure Obama (whom, remember, I do not support) is regarded in the world as 'weak' and 'ineffective'? I don't get that impression. He is very popular in Europe, for example.
As for China taking over all Korea, I think not. It would not look good to the countries with which it is trying to develop long-lasting commercial and political ties (view Africa). Becoming imperialistic (again) would be injurious to their campaign. Better that Korea unify itself and then become closer to China and further from the sphere of American influence.