Oh and the town where Martin comes from is right next to Berlin-Zehlendorf, (which was/is West Berlin). So not in the sticks.
The house his mum lives in is in Kleinmachnow, and they are exactly like that! It was a desirable pre-war small-villa kind of place, but after the wall went up it became a bit of a backwater, because it was quite cut off from its natural hinterland, iyswim. You had to go to Potsdam and then to East Berlin, which would have been quite a circuitous journey.
I wondered about accents but assume that a Potsdam/Brandenburg accent will be similar to Bielefeld (or maybe the soldier he replaced was from the Berlin/Potsdam area originally but stationed in Bielefeld, not sure if they said). If Martin had come from Saxony there's no way he could have passed for a West German.
I think the accent thing is fundamentally implausible, tbh. Potsdam/brandenburg accent would be not that different from Berlin, but significantly different from West German accents. In the film they get round it by making them all speaker Hochdeutsch, which is the Gm equivalent of RP or BBC Egnlish. But tbh IRL he would certainly have been outed through linguistic slips, not just accent but different different words and turns of phrase - he would have inadvertently said Plaste instead of Plastik, or Broiler or Konsum, or failed to understand a Western slang word or colloquialism, and it would have been game over.
But it's a good watch, for all that.