Actively toppling the North Korean govt would still make china very very unhappy, and the world economic costs of pissing china off would be very great.
Given NK's limited finances, they would be unable to wage a sustained campaign, and it would not be in china's interests to provide monetary or material support for such actions. (China really still needs western financial backing to continue its heavy infrastructure overhauls it needs to become a 1st world power in the full sense.) If NK jumps the shark and bombs the bejeebus out of SK, (and thus pisses of the UN, and UN allied nations) china will be politically obligated to tell glorious leader to put his nuclear dildos away, or have his fat little bottom spanked.
Unfortunately, this position with china is temporary-- once china reaches said level of industrial and economic independence and empowerment, it wont need to pussyfoot around the issue anymore. Things can get very uncertain once that happens. I dont dare hazard a guess.
The best solution to the NK problem is to find ways to pierce glorious leader's cult of personality, and to inundate the North Koreans with counter-propaganda, so that they do an "Arab Spring" type thing. Internal civil instability will make it impossible for NK to conduct any kind of external violence, and stands a good chance of destroying their already resource starved government.
Why do you think Glorious Leader tries so hard with his Potemkin villages, profane quantities of insane propaganda, with a heavy frosting of gulags and totalitarian interment camps? He is absolutely paranoid of internal civil rebellion. (if he isnt, his military cabinet most certainly is.)