Regards, Ivan
I'm not sure the Peoples Republic would cotton to the Japanese and the Manchu-kwo stomping around on the Peninsula. That harkens back to some ugly days not so long ago, days that Mao and the Glorious Revolution exploited to their considerable advantage.
But I would certainly think the Russians and the South Koreans, as well as the "co-prosperity sphere" you mentioned, could bring some pressure to bear, possibly even with China's surreptitious support. It could be a good imperial move for China to annex North Korea, depose Kim, and bring a Khan-like peace to the region.
I don't know that Japan or Taiwan would care; North Korea in the hands of a raving lunatic like Kim is more dangerous than another province of China.