The correct answer is that a windmilling propeller produces more drag than a stopped propeller. You don't seem to understand the question.

Are you attempting to say that an aircraft with a fixed pitch "cruise "prop" produces the same drag as the same aircraft with the propeller stopped? Feathering isn't possible in a fixed pitch propeller, but stoppage of the prop is.

The windmilling RPM of the propeller depends upon it's blade angle, but also upon the airspeed. The higher the airspeed, the faster the windmilling RPM. You seem tied to the concept of a fixed pitch Cessna 172, which is a slow airplane anyway. If you don't understand the relationship, see what the RPM of the 172 is in a dive at idle at Vne vs. a descent at Vx or Vy for the same airplane; the faster the airflow through the windmilling propeller disc, the faster the RPM, and the greater the drag rise.

Your 172 at idle on the ground can achieve a prop RPM as low as 600-900 RPM, while in flight may seldom be seen below 1200-1500 RPM, even at low speeds such as final approach. Why? Airspeed. At higher speeds, there will be higher RPM's and there will be a greater drag rise from the windmilling propeller.

The drag on a windmilling propeller will always be higher than a feathered or stopped propeller and more than one type of drag exists. It's not just aerodynamic drag at the propeller blade, induced drag, but also increased parasitic drag as well as the drag of moving the propeller against the resistance of the engine. On that account, you don't seem to understand how significant the internal drag is in the engine, or that much of the operating power of the engine itself is used to overcome its own internal resistance. Once the engine is no longer driving the propeller and the slipstream is, all that internal resistance and the energy absorbed by the slipstream performing the work of moving the engine and propeller (and accessories) is all drag.

Altering the blade angle will make a slight difference, if you can alter the blade angle, but remember the discussion is about a windmilling propeller vs. a feathered propeller (or stationary prop for your fixed ptich that can't be feathered). As you move off the low pitch stops, you're moving toward a feathered position. If you're saying that a propeller closer to the feathered position produces less drag than one on the low pitch stops, you're correct (given a constant airspeed value), but this belies your point. In fact, you're making the opposite point; a windmilling propeller produces more drag. The amount of drag depends upon blade angle and RPM and airspeed (RPM in a windmilling state being a function of airspeed, for a given blade angle)...but a windmilling propeller will always produce more drag.

If you move away from your 172 to a higher performance aircraft operating at higher speeds (to which end Aerodynamics for Naval Aviators was addressed), the drag rise can be so significant as to render the aircraft uncontrollable...that's the difference between feathered and windmilling. You'll also recall above that ANA points out that the drag rise can be high enough to cause structural failure of the aircraft, particularly the vertical stabilizer and attach points.

That's pretty damn significant, and hardly a red herring.

Next consider where many constant speed propellers go when oil pressure is lost (not uncommon in an engine failure, especially an engine failure in which oil is lost...I've had a number of those). Many propeller installations revert to the low pitch stops under spring and/or nitrogen pressure, and begin to act as a fixed pitch propeller; RPM varies with power setting and airspeed.

There's a reason that turpopropeller engines fail to the feathered position, or are supposed to and move to a higher pitch, reducing windmilling drag until the propeller can be or is feathered by pilot action or by natural consequence. Until that time, regardless of whether on the low pitch stops or at an intermediate position, the windmilling propeller will still produce considerably more drag than the feathered prop.

In what aircraft and under what circumstances have you observed this?

It's absolutely not a "red herring," and your assertion is supported by no documentation. In fact, it's not supported by your link to ANA, either. I can tell you that I've flown turboprop installations (as noted previously) that produced such significant drag at an idle windmilling state that I was physically thrown forward hard in my shoulder straps and had to follow the stick through forward to nearly the vertical to keep from stalling and experiencing a control loss, due to the rapid drag rise. As ANA noted, it was very much like throwing out a parachute, and breathtakingly dramatic.

If you've ever gone from a windmilling descent to a feathered descent then you'e experienced the significant reduction in drag from the feathered prop, vs. the windmilling state. There's really no way around that.