Doesn't matter, Japan will only have to set it's sight on Japan or NK,... China on the other hand would have to choose carefully which country they would want to spent their arsenal to, the list is very diverse, from Taiwan, Japan, SK, US bases in Guam and the Pacific, Philippines, Australia,Indonesia, Vietnam.
The very idea is to strike China's port and assembly points, your 1.4B advantage in manpower will be cancelled immediately once there's no ships to transport them or warships to escort. This will be very true in the case of an amphibious assault on Taiwan and Japan.
are you sure all the countries you mentioned will risk an oblivion for the love of a third country? Trust me in case of a full blown war each and every country except the one at war will be sending secret messages to China.