"So what to try now?"
This may be too simple, but if the rear master cylinder is good and will not move under force and the rear brake shoes are not against the drums at rest, then perhaps your rear wheel cylinders are frozen. Cars that sit have this problem. My new wheel cylinders with fresh brake fluid (meaning low moisture) were frozen after sitting for one year.
The pictures indicate to me that the rear master didn't move at all, which indicates a hydraulically locked system.