I think the questions/mysteries you raised are mostly solved by reading the works on Anthony Sutton.
https://www.amazon.com/Wall-Street-FDR-Antony-Sutton/dp/1905570716
https://www.amazon.com/Wall-Street-Bolshevik-Revolution-Capitalists/dp/190557035X
https://www.amazon.com/Wall-Street-Hitler-Antony-Sutton/dp/0945001533
Follow the money.
A forensic audit of Marshall would answer all the questions.
In "America's Retreat From Victory" he obliquely points at something:
"...After Colonel Marshall had been removed from command at Fort Screven, he left for Fort Moultrie in South Carolina. The residence of the Commanding Officer of that post was a large, rambling structure, replete with 42 French doors opening on two verandas. Mrs. Marshall, as she reports it, had barely provided 325 yards of curtains for the French doors when orders came transferring her husband to Chicago as senior instructor of the Illinois National Guard...
I see this as a clue, that he (or his wife) had a grasping nature, and were susceptible to the pressures of money. We were in the grips of the Depression when his wife was purchasing 325 yards of curtains for the commandant quarters on Fort Moultrie.
I have never fully been ready to reconcile Marshall as a Communist or even a closet Communist, but I think it is possible influence was being negatively asserted on him in the form of blackmail of some kind, financial, sexual, et.
And that is something the Communists were very good at. And once they own someone, there is no escape.