Forgive if my quick scan through has missed something on this interesting thread...will read again later. But for the moment.
If an aircraft was suspended on a string in a total vacuum, but magically made to run for the purpose of measuring torque, what would be the effect of the forces then?
Clearly, we would only be considering accelerating masses, but this may serve to clarify some of the basics.
On the subject of props being driven by the gas-flow only, surely, all one is doing is changing the coupling from molecules of steel to those of gasses.