Plain and simple, I don't know. One always invokes stress and psychology with restraint, but after all the evaluations and workup and treatments you describe, I don't have any other ideas. My approach would be to treat the symptoms with topical steroids, anti-itch lotions containing camphor and menthol, and antihistamines. I realize these have been tried, but there are various examples in each category, and sometimes one works where others have failed.
Sorry I can't be of more help.
Dr. Rockoff