Declaring war on Japan, alone, doesn’t have any consequences. Most games begin with that DoW on R1, even if Russians don’t move into China. Its attacking the Russians that implicates the Mongolilan Rule. Then, when the US enters the war, they can land, there is never a situation where you cannot do this for practical reasons, just losing the NO.
Back to the original discussion, its a great rule as it is. If you can keep Russia standing on its own, do that. if you need it as a base, do that.
SZ 125 is well contested, so is archangel, both make this bonus moot, by G3 you’re usually burned and then the allies can come in either way and nothing is lost.