China cannot withstand all out Russian first strike. Russia is too big to be completely hit with the current size of chinese arsenalI don't understand what you are talking about. China has never had any claim to the Kuril Islands at all, unlike Japan who previously owned them.
And China could wipe out Russia too. MAD flows both ways. But make no mistake, the possibility of China going to war with Russia is about as high as going to war with North Korea, which is basically zero.