In HOI3 you could take China out without having to deprive the navy. Pretty much the war was over in 1939 with plenty of time to prepare for USA.
On taking out China, depends on if its single player or multi player.
If multiplayer, you main goal is to force USA and UK to throw things at you rather than Germany. Ditto on war with Russia, everything aimed at you is not aimed at Germany.
Never seen Japan with a manpower issue.
I have had manpower issues as Japan but I always pumped out lots of cheap infantry, marines, and mountain units until I had a hard time supplying them. Then I would try to use that early large army to grab everything I could before my enemies built up. This eats a lot of manpower and supplies.