I have tried tens, heat, ice, acupuncture, botox and stretches of course but the only thing that worked slightly was botox.
It eased the piriformis band of bum pain fantastically, but didn't do a great deal for the nerve pain, so the conclusion is that scar tissue, which the botox can't relax, has hold of my pudendal, sciatic and possibly post femoral cutaneous nerve. I did have a lump (probable heamatoma) on my P muscle after a fall which seems to be a classic way for scar tissue to develop there. A physio that I saw a couple of years ago was treating a guy who'd had PS for about 40 years, she could feel the spasm'd bumps in his muscle, but managed to treat him over a number of months (heat/massage etc) until he was almost fully recovered. Unfortunately I didn't respond similarly. TBH I didn't respond at all! except in pain.
The acupuncture was one of the worst things I have experienced. Just the needle into the muscle. . . it simply would not go in at first and then wouldn't come out again! almost thought I would have to stay with a porcupine'd piriformis!

Just the fact that P muscle stretches set your pain off seems to be a pointer in that direction, definitely worth exploring; and as you say one muscle is smaller than the other which can be a classic PS symptom too. That's not to say that you couldn't have PN further down the nerve (sorry).
I guess a Potter scan could give you some answers. A botox shot into the muscle could be a diagnostic tool either, to see if it helps anything, sometimes that can be a cure (so they say)
Anyway I wish you lots of luck, and hope you get some answers.
take care,
Helen