Those two short electric trim-up commands at the end of both crashes just before enter info a dive keep puzzling me... Farfetched maybe, and probably repeating what others have suggested already, but what about:
E: The electric trim didn't move the trim wheel at all. Flip up once, no trim wheel movement. Try it another time, still no movement. The pilots conclude electric trim it isn't working and proceed (and the MCAS steps in again).
SLF