I can only imagine what it must feel like to be itchy all over! And definitely can see how plucking starts rewarding the pain centers even when it hurts, just like picking for us or chewing for dogs.
My vet didn't see any evidence of a current yeast infection either on his skin or in his mouth or crop, so I really hope that is in Cotton's past! He's starting to get more downy feathers now and has sprouted a few shoulder feathers (shhhh don't tell him!). I also rearranged his cage a little bit after his vet visit when I cleaned it, because I think hitting his feathers when flapping might have been part of why he started snipping them off. And he'd left his tail alone until he fell off his perch in the car, so maybe he bonked that too. I'll have to be more vigilant about how he's using his space if he lets his feathers keep growing.
He might already be a feather chewer/plucker out of learned habit, of course. My first dog had chewed the skin right off his tail, and it took a couple years of intensive medical intervention and constant e-collar wearing to prevent chewing. So far the vet wants to just see how Cotton does with transitioning food and continuing to fight his respiratory infection before trying anything more substantial for plucking, especially since he's still terrified of being handled.
He beats his wings quite a bit and it's sooo cute! I felt bad the other night when he was trying to get down off his cage because he kept trying to fly - but hopefully that means he does know how! My vet said his chest musculature was well developed so fingers crossed he'll be able to fly again some day.
At night lately since it's been cold again, he has a room heater, the Sweeter Heater on top of where he sleeps, a blanket covered with a sheet for darkness over 2/3 of his cage, and a humidifier. My plants in that room are very happy