Given the parameters you've set, the Russians would've taken cared of the Japanese troops if Japan invaded Russia.

Why do I say this?
The Imperial Japanese Navy might be able to transport and supply troops while these forces are still near a port or beach. However, the Japanese didn't have that efficient a logistical system to continue supplying their troops further inland. Most probably, the Japanese would've ended up using infantry as pack mules to transport supplies, which would be a very inefficient method given the size of the invaded land and the force opposing it.
The Soviet Navy would've been sunk, I agree but the Japanese ships would be of very very marginal use once the Japanese troops are deep in land. What would be needed by that time would be more transport and tanks.

Bottomline, a Japanese invasion of Russia given your scenario would've lengthened the World War II timeline but I believe the Russians would still manage to defeat the Japanese invasion. But defeat Japan as a nation, I doubt it. The Russians wouldn't have the capability to carry the fight to Japan while still being engaged by the Germans.