If they capture an original Russian tt from the Central Powers, is it “liberated” back to Russia, or can the Allies take control of it?
This one is actually really interesting; it’s the method by which the Allies can concievably trap CP units in Russia.
The territories the Allies take in Russia will always go back to Russia since the Revolution will never happen if Moscow is under CP control. Since the capital is not under CP control, it would be Russia that gets the territories.
What about contested tts: if the Allies defeat all CP units in one of these, does the tt also revert to Russian control and therefore becomes off limits for the Central Powers?
By the same reasoning above, yes.
Do Russian mines still have any effect?
Interesting, the rules say Russia signs an armistice, but nowhere does it say in game terms that Russia is no longer an enemy of the CP, it just changes how they interact. It seems according to the rules as written mines must still be on.
Do “contesting” Russian units count as friendly to the Allies for the purposes of moving to/from contested tts?
“rules restricting land unit movement out of contested territories no longer apply to Russian Territories”
Remember though; you can’t normally move out of a contested territory to one that is merely friendly, it must be to one controlled by YOUR power. So, for example, if Switzerland is contested, Germany cannot move out of it into Tyrolia even if Austria controls it (I don’t like this at all, btw).