From: Aarhus, Denmark
The briefing states that if the Germans withdraw, the free unit can advance into a vacated hex, thereby releasing the rest of the army. The hex to be advanced into must be the hex in front of the free unit, or can it be any hex along the front of the army concerned?
It can be any hex along the army's front and it can be any unit that enters it; even from another army.
Also, do the armies become mobile automatically at some point, or, if the Germans do not withdraw, is using the theater option the only way to activate them?
If neither the Germans or Soviets do anything to activate, their formations can in theory be in garrison state for the duration of the game.
Both players need to consider a number of factors. If the German player choose not to activate any of his army corps, he risk having the soviet offensives on the flanks cut them off. If he pays to activate a corps he can do a full withdrawal, but the Soviet player saves the VP of activating the opposing army himself. The Germans can instead move just part of the units away to help on the flanks, but not leaving any open hexes. This way the Soviets will still have to pay to activate.
However even with no hex having been abandoned, the Soviet can, by moving units from elsewhere, or just with the local penal battalions, start an offensive with in the army's sector, have the army activate at no VP cost once a hex is captured.
It's sort of a game of chicken.