"Power doesn't achieve lift"
Try telling that to a helicopter pilot.
It does not directly achieve lift. However lift cannot be produced without power, and power produces lift by causing net airflow over the wings. This is achieved in various fashions (I can think of 4 off the top of my head), although for now we are talking about by its use to produce thrust to overcome drag and keep the aircraft with fixed wings moving through the air.
Unless you really are not keeping up with the physics here you are quite aware that my words were short hand. Do I really need to go into detailed, pedantic description, like the previous paragraph, of every idea every time I mention it, even when I have already explained it before?
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).
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.
Using a back-of-the-envelope calculation, simplified but with either conservative or realistic assumptions to either constrain or approximate the answer to a problem is a common technique in physics (conservative assumptions to constrain, realistic assumptions to approximate). I remember at university for example being shown that you could work out in a few seconds how long it takes to boil an egg, from basic priciples and the properties of the egg. It is not meant to be an accurate representation of heat flow in an ovoid shape and does not even attempt to model the denaturing of albumen proteins. However it comes up with about the right figure (I think we got 5 minutes).
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.
Likewise how is a "large four engine airplane with radial piston power" relevant to most light aircraft?