There is another cause for pedals dragging, too tight chain. But yours has noise...

So, here is my experience:

The 3sp AW that came on my R20 ('76) I have never liked much. It has those same problems, dragging pedals and noisy back-pedal. I have tried everything without much success, but I *think* I know what the problem is.

You mention undoing the RH cone a half-turn from finger-tight. That may be the problem. Undo it 1/4-turn only. What I *think* happens is the inner works rubs on the shell at the RH side from too much play, producing a distinct rubbing sound. So have another look at the cones and adjust especially the left one with as little play as you can, allowing the tiniest amount of knock at the rim that you can get. This reduces the problem by pulling the inner works to the left as much as possible. But in my case that improves it for a little while only. Very frustrating.

As I said, I am not sure about my diagnosis, but it's the only thing that remains after eliminating all else. Another much older hub I have has none of these problems.