RE: prop spinning too fast?
I can barely fathom the idea of the planes' speed contributing to the overall speed of the prop. If this is so, the explanation that makes the most sense to me is that the prop tips have more air molecules blowing past them on a moving plane than on a plane that is parked.
I should be able to visualize this same addition problem in reverse where the rotating prop speed contributes to the planes' speed......not from a view point of prop thrust, but of the props' rotational speed. I guess what you are saying is that without knowing what the thrust is, there would be too many unknowns?