The flap will be fine. It is the propeller I am concerned about.
A propeller directly in the wake of a wing, with the propeller axis in the wing's plane, will hit the boundary layer of that wing each time the propeller blades cut through that wake. This means a sudden decrease and increase again in dynamic pressure which results in a sudden change in bending loads on this propeller blade. This will
- reduce the lifetime of the propeller due to material fatigue, and
- produce a distinct noise at twice the propeller frequency for a two-bladed prop (and at six times its frequency for a three-bladed prop).
It will be better to place the propeller ahead of the wing or aft of it with some distance between the trailing edge and the propeller plane in order for the wake to dissipate and for the dynamic pressure change to become less sharp.
And using several wings stacked vertically has turned out to be a poor solution maybe 100 years ago already.