The movable propeller shaft was not the problem I was thinking of but the propeller diameter. You are right about the "high-revving" ones but were they capable of something like this in the 30s? AFAIR this a/c never flew at all. That's the reason to ask if somebody knows more.A small, high-revving propeller can do the job. Not as well as a large, slow-turning one, but still manageable—and probably just enough to get the aircraft airborne and stable.
As a reminder, some Dornier seaplanes also featured movable propeller shafts.
Cheers!