How would it be possible for a communist group to control Germany, with it starting sometime between WW1 and WW2?
Yes. IMO, you could see communist Germany and Hungary if the luck falls a different way post WW1.
Personally I think the Bela Kun Hungarians went a little nuts, I don't see them having much a chance.
Now I'm contemplating WWII as a well organized Trotskite Germany & central Europe vs Stalinist Russia.
Trotsky was also a Leninist.Then I suggest you do away with Luxemburg and Liebknecht - despite the Trotskyites claiming them as their own they were more and more falling into the camp of Lenin (and by extension Stalin) theoretically towards the end.
Trotsky was also a Leninist.