The way I see it, if the ROK and US engaged the Norks, the Norks would lose, of course. But PRC would intervene near the end of it and again, drive the US back to the 39th parallel, thereby giving the PRC a buffer, once again, from the US/ROK. The PRC hates the Norks like everyone else, but would never let a US ally right on the PRC border.
Why would Red China throw away 40 years of modernization and increased trade just to save a tiny country? Who will feed the hundreds of millions of Chinese thrown out of work?