How does one produce lift without power?
Power is required for autorotation, otherwise one could autorotate and remain level. I suggest you find a physics text book and look up the meaning of the word power. I can assure you it will talk about conversion of energy. Energy can be in the form of potential energy or in the form of chemical energy before its conversion, but power is by definition simply a rate of conversion of energy.
To try and simplify things for you we'll take a glide speed of 70 kts. At a glide ratio of 3:1 that means a rate of descent of 2363 feet per minute. That is not a rate i would like to see in a light aircraft.
Perhaps you could, instead of trying to nitpick minor factors in a calculation that was never meant to be accurate, factors that are overwhelmed by the scale of extra thrust needed to lift your 3:1 L:D ratio aircraft, you could suggest one that has that ratio.
I have looked up the C152 again, and the estimate I found was 8.75:1 (pretty close to the 9:1 I mentioned earlier). So where are your examples of "most" light aircraft at 6:1? Where is the normal light aircraft, relevant to private flying, with a 3:1 lift to drag ratio?