There is the argument that the "natural" accent for the portrayal of an educated European of the early 20th century is Received Pronunciation English, as that is what they would be speaking (with a possible hint of their own accent) if they were speaking English. Language teachers teaching Germans to speak like Tom Cruise being scarce in 1920s Germany.
This is why educated Romans and Nazis were routinely portrayed in the movies by RP-speaking Brits, and I don’t have any problem with that.
The riddle presents itself when trying to map non-ruling class accents from the film’s subject area onto English-speaking ears. In Oliver Stone’s “Alexander,” Stone notably attempted to distinguish between Greeks and Macedonians by giving the Attic-speaking Greeks the RP-accent and having the Macedonians style various interpretations of Scots accents, to imply the “northern quasi-barbarian” status the Macedonians held in the eyes of the Greeks.
Unfortunately, the American audiences were only puzzled by this, and the British audiences (as is their wont) focused most on debating whether or not the various actors did creditable Scots accents.
Moving all of this over to Germany, the situation would be even worse. Some argue that Bavaria is essentially Germany’s Texas, inhabited by loud, proud and religious hillbillies holding loyalty first to their own homeland, and then, secondarily, to Germany overall. Would it have been advisable to give Stauffenberg the accent of an educated Texan?