"Power doesn't achieve lift"
Try telling that to a helicopter pilot.
Would you prefer to have a rotor wing discussion instead? Not at all relevant here. However, power is not required for a controlled, autorotative state, either. Power in a fixed wing airplane is not at all required to achieve lift. Try saying otherwise to a sailplane pilot.
However lift cannot be produced without power, and power produces lift by causing net airflow over the wings.
Entirely untrue.
Yes a windmilling prop provides prop drag. However an engine has to overcome prop drag in order to provide thrust, that is one of the inefficiencies we both alluded to. Therefore it is on both sides of the equation and cancels out (in fact it saps thrust in the powered aircraft by more than the increase of drag after engine failure, as the prop is driven faster).
No, not at all. A propeller producing drag in a windmilling state alters the LD ratio substantially such that the glide ratio of an aircraft isn't comparable with a high drag propeller and ingine installation to a situation in which the engine is driving the propeller. The drag incurred, even that imparted to the engine (which does not contribute to airframe drag) in a powered situation, is considerably less than windmilling drag. This is why we feather propellers. Two examples were given of this demonstrating the effects of high drag from a windmilling propeller, but based on the comments at the end of your post, this seems to have gone over your head.
However all this proves is that you are over complicating the matter. I was never making any attempt to precisely calculate thrust or maximum lift. I was using figures of the correct order of magnitude but deliberately conservative assumptions to simplify the problem to prove that there is no way that an aircraft with typical power to weight ratio of light aircraft could fly level if its glide ratio was 3:1. This I have adequately shown. You are simply nitpicking, and have not addressed the central point.
You did attempt to do this, but you were shown to be wrong, as I demonstrated in the case of the Dromader...which experiences a 1-2:1 glide ratio at idle...yet miraculously manages to fly all the same...even at greatly reduced thrust. You didn't find this relevant, for some reason....perhaps simply choosing to dismiss a personal example which shows your point incorrect.
What is the relevance of an M18 Dromader to "most light aircraft" that you first posted about? That is a piece of drag-inducing ironwork with a huge engine on the front and a massive prop. It doesn't have the power to weight ratio of a Cessna 172, so of course my scratch calculation is not relevant.
The M18 is a light, single engine, tailwheel general aviation airplane in common use throughout the world. I used it as an example because I spent seven years flying them. It serves as an example of an airplane with a very low glide ratio (in some of the copies I flew) which is very incongruous to the powered L/D, with the vast majority of it's drag in a glide coming from the propeller. It demonstrates that a low glide ratio in a windmilling state, with the propeller and engine absorbing energy from the airstream, cannot be used to determine how the airplane will fly or on what percentage of power. The fact is, it can be flown at very low power settings, certainly not needing it's rated power to fly...despite having a very, very low glide ratio. In fact, it has nearly the same power to weight ratio of a 172, now that you mention it...so yes, it's relevant.
Likewise how is a "large four engine airplane with radial piston power" relevant to most light aircraft?
I spent several years flying it, and am familiar with it, so I included the example. As a propeller driven aircraft with only one out of four engines windmilling, with three others producing takeoff power...the airplane at times couldn't maintain level flight. It's an interesting study in the effects of windmilling drag in a real world situation...even at twice the power to weight ratio of the 172. The glide ratio can be very low, which doesn't necessarily have a bearing on how much power is required to fly in a powered situation.
You seem to miss the point, or even make light of it by suggesting that the drag which causes the low glide ratio has an identical effect in robbing engine performance such that the glide and the powered level flight or climb are handicapped in the same manner. This is simply not true.
For the original poster, again, the fact remains that the glide ratio is really unimportant; it's the landing at the end which counts, and once more, this is entirely within the purview of the pilot.