"issue at hand -- like the coupling of this prop with an engine that
produces high harmonic vibrations. The O-360 lacks a counterbalanced
crankshaft and that fact is responsible for the red arc in the
upper-middle portion of the RPM range. He seemed to think that the
biggest factor is the fact that most people assume that their props
are
"smooth enough" and don't need dynamic balancing when in fact they
could
benefit from it."

Dynamic prop balancing won't reduce prop stresses very much. It will
reduce the vibration transmitted to the engine and eventually to the
airframe via the mounts & cowl etc, but not the hub or blade stress.

Prop blade and hub fatigue is caused by the constant torsional
pounding of the engine combining with a torsionally resonant
crankshaft and propeller blade system. On some engines there is a
dynamic absorber at the back that substantially reduces the resonant
buildup of crank and propeller stress, but many versions of the 4
cylinder IO-360 Lycoming engine don't have these absorbers. Instead
they placard the tachometer and expect the operator to not run at
certain subharmonics of the crankshaft-propeller torsional resonance.

To understand how this torsional vibration mode operates, you must
imagine being an observer sitting on the spinner while the engine is
running. When this mode is excited, you would see the prop tips
oscillate to-and-fro while the rear of the crankshaft oscillated fro-
and-to. There is a lot of high frequency (about 220 Hz) torque going
through the prop hub and that's what this is all about.

Most mechanics don't understand how the dynamic absorbers (they are
not counterweights) on the crankshaft if included, are supposed to
operate.