you have 3 territories that do not implicate the mongolian rule, so if the russians depart, you can always take those with extra transports without ever comitting to a major invasion
if the russians stay, or add a fighter, they can tie up some minor forces and, at a critical time, lure japan into a diversionary attack that keeps them up north.
eiher way, the money is down south, not up there. Still, you can take it all, provided you dont dilute the japanese fleet or money push