I am sure that the Koreans are very nationalistic and I doubt they will side with the japs against China. They hate the Japs more than the Chinese hate the latter. But, if they become united, they might want to ally with a distant superpower to balance off China-- very much like Ukraine is trying to do against Russia. So, until China has unquestionably, and unambiguously, surpassed the US of A in wealth and power, it is far safer to keep the 2 Koreas disunited.