On the one hand, most all of the conspirators were not Nazi Party members.
On the other hand, all of the conspirators were indeed human.
Or did you just mean: “The only good Kraut is a dead one?”
It occurs to me, on the subject of accents, that it’s really a no-win situation. Germans don’t perceive themselves as having “German” accents, they perceive each other as having Bavarian, Berliner, Plattdeutsch accents and so forth.
Having English-speaking actors with different English accents should only be a distraction to those who think the whole film should have been in German with English subtitles.