It's realistic. Just ask the Baltic states (invaded, colonized), Finland, Austria, Poland, Czeckoslovakia, Georgia, Ukraine, Afganistan. I'm sure I'm leaving out a few. Or, just compare the German experience to occupation by US/UK/France on the one side and Soviet Union on the other.
The possibility of an invasion of someone isn’t unrealistic by a Russia, but an invasion of Norway specifically not particularly realistic by the Russia of our timeline. For one, Finland and NATO-member Norway would have much stronger backing by the western allies than they did in 1939. For another, nuclear states would have to give up MAD in favor of Mankind Absolutely Rejects Nuclear Explosives.
