This is a scenario that's becoming increasingly implausible, not in the least because North Korea and the PRC are becoming increasingly hostile to one another. The PRC has said that, while they will support North Korea in a defensive war against a United States-sponsored South Korean invasion of the north, they'll abandon their support of the DPRK should they be the ones to start the war. I feel like the only way this would work is this - the United States and NATO abandon their support for the Republic of China (Taiwan) and recognize the People's Republic of China's claims to Taiwan, and in return, the PRC withdraws all support for North Korea outside of humanitarian aid. This, combined with assurances of non-aggression between the PRC and the NATO powers, would mean that NATO and South Korea can finally enforce their demands for more human rights and less nuclear testing on North Korea without the threat of China helping North Korea. Either North Korea is forced to back down; they go too far and start a war, either by making a declaration or forcing South Korea/the USA's hand, and without Chinese support North Korea is defeated (and maybe goes nuclear, but let's not go there); or a United Nations-sponsored invasion of North Korea begins in response to repeated North Korean human rights violations and threats of war.
An arrangement like this would probably strengthen Chinese-United States relations (at the cost of permanently catastrophucking Taiwanese-NATO relations), and the ultimate result would be a united, pre-1911 borders People's Republic of China, a united Republic of Korea under the South Korean government, an end to the Republic of China/Taiwan and strengthened PRC-NATO relations to an extent.