Actually, imo a nuclear armed but fully independent Japan that has kicked out the US occupation forces is better for China, that having the US occupation forces at China's doorstep and controlling the meek Japan of now. There is some very bad history between the two countries, it is true, but if both side are willing they could get over that and strike a mutually beneficial balance. Asia for asians and all that. Besides, no harm in directing the dormant samurai spirit towards the Pacific and payback for Hiroshima and Nagasaki, to the regretable detriment of US hegemony.
For the same reasons, a united nuclear armed Korea is better for China than the current situation. Anything that drives the US military forces away from China's border is better than the current situation imo.
Imagining for a minute this scenario might be possible, you have for instance the 3 major shipbuilding powers of the world on more or less the same side against the US. The US will be finished as a Pacific power then. This not to mention the overall economic factors, on one hand China dwarfs both SK and Japan economically and it will continue to do so even in the best cases scenario for SK and Japan, but on the other hand, since Japan's economy growth is suffocated by the US boot, without that boot their economy might explode with growth, maybe just like in the good old days of the 1980s. Which is good for leaving the US further behind economically and good for cementing Asia as one THE major and biggest world economic pole. Multipolar world and all that.