If they decline to attack Yunnan, then you can join with UK/ANZAC also.
Once China has the 6+ every turn and some artillery, Japan has problems
Japan can sacrifice planes, but once its lost 4-6 of them, the odds for Calcutta Attack/Sea Attack decline
China can’t do much by itself but there is a huge difference between a dead China of no threat and +12 Japan Money and having something alive and threatening
Now compare that to the situation where China, Russia, Uk, Anzac are all chomping at Japan’s inland territories, worth 20,
He only has two choices; spend his troops to push you back (this is a wash)
Or pursue his strategy elsewhere and you clamp his empire down and take his $$
However, the truth is that Japan chooses these outcomes for you. If he confronts US or AUS, you can play on the Mainland. If he moves his fleet towards SZ 37, you have be much more careful. He can take the spice islands AND hold the little powers off but he cant do both of those things AND fight the US 1:1