Japan will eventually lose a naval engagement with America, no questions asked. Question is, can they break Russia before America breaks Japan (ie reduces Japan to no navy, you don’t need mainland Japan here, just destroy their fleet and you beat Japan.)
If Germany built a Carrier, then it is time to go after Japan. It’s the most bone headed maneuver in the game to build a carrier for Germany, IMHO. With that Carrier, you can fully expect British and Russian forces to own the eastern european states for quite some time freeing America to run roughshod over Japan without much of a worry.
If Germany did not build a Carrier, then significant resources are going to be needed to slow Japan’s mainland down. A mere 2 tanks, 4 infantry from Russia can easily do this. (That’s in addition to the forces already in position. Russian fighters in range, but not used against Japan, just ready to be if needed.)