About your HR, Russia is merely strong enough to protect Moscow, not talking about some far east adventures.
I guess one way to handle this would be for the HR to say that Russia can declare war on Japan after two conditions have been met:Germany has been defeated Russia has had time to transfer many of its forces from Europe to the Far East
This is basically what happened in WWII. Stalin had an agreement with the Americans and British that he would declare war on Japan three months after Germany’s defeat, to allow him time to shift his forces eastward. Which he did.