China would not move unless there is an unmistakable sign that its economy or national security is at significant risk due to N. Korean crisis.
That is, either the West is squeezing China economically(financial sanction, raised tariffs, or relocation of production facilities)
or Japan is serious about scrapping Article 9 of Japanese Constitution and expanding military power.
Another case is that N. Korea creates unacceptable provocation, such as attempting nuclear test or trying to shoot at U.S. or Japanese (or even S. Korean) military.
China must be confident that it has great intel sources and connections in N. Korea, so that they can smell trouble the moment it develops. I am not sure if such strategy is still wise. N. Korea has become brittle. It may look impenetrable and hard. However, with instability rising, a few coincidental domestic events can spin N. Korean situation out of control, before China has the time to react properly. Of course, some 150K to 200K Chinese troops stationed across N. Korea's northern border could be used to handle such a situation. However, it could complicate the situation by provoking response from U.S. and S. Korea. Most S. Koreans won't tolerate PLA moving into N. Korea en masse. S. Korea and U.S. will move up across DMZ to beat China's rush to Pyongyang. It could become pretty dangerous. Not only militarily, but also economically. Chinese economy could go south.
The surgical palace coup by pro-China faction would be ideal for China, but I am not sure if they can control the situation once the safety pin is finally removed from the grenade called N. Korea.
Maybe that is why China is dragging its feet. China does not think it can control the course of events once Kim Jong-il is gone and the tumultuous change ensues. Since they are not sure, they just drag out situation in N. Korea by providing some economic aids, until Kim Jong-il finally leaves them no other options except to remove him.
Why tinker with something they do not know what to do about? Buy time and just hope that they get lucky. That is my take.
Better to keep the devil you know than the one you don't know...