In the resistance movement section, you list Upton Sinclair and Norman Thomas; how about adding Robert Heinlein, who was Sinclair's press secretary during the latter's run for gov of California? This would fit right in with Heinlein's fantasies of being a resistance leader, which he expressed in several of his books (like, If This Goes On--, and The Moon Is A Harsh Mistress).
As to the military governors you list, I think you sadly underestimate the loyalty of these great Americans to the U.S. Constitution; I don't see a single one of them going along with a coup d'etat, not even McArthur. Indeed, I can envisage McArthur leading the wing of the military that rebels against the coup leaders--he would see it as the greatest "performance" of his life.
Since the army was so small at that point, it might be possible for armed civilian bands, joined by lower level officers with ultraright politics, to take over. If they seize a couple of military bases by surprise and have a lot of World War One vets in their ranks, and immediately seize the capital and arrest Roosevelt, they might have a change of winning.
On officer who plausibly would have joined such a coup? Charles Willoughby (McArthur's intelligence chief and "pet fascist" during World War Two and Korea). On the other hand, Willoughby worshipped McArthur, so (assuming he would have met McArthur earlier than in OTL) even he might have opposed the coup.