My Quaker Pickles is a plucker too. Doesn't sound a severe as your bird, but I started him on red palm oil from Harrisons, let his flight feathers grow back in, got a better humidifier, and let him and my other Quaker spend more time together and he has stopped plucking. Before I rehomed him he lived in the same cage with another Quaker. They got separated, Pickles broke his leg, and he became very wild. He's a great talker and sometimes will let me touch him but he was almost bald on his chest and was working on the back of his neck and his legs. The red palm and humidifier have soothed his skin and I think he misses being with another Quaker so I let Buttons hang out with him and he seems much happier. I know our situations are different but have you tried red palm oil for your little green chicken? Pickles is pretty wild too. I think they go through phases as they mature and if we can be patient and consistent, eventually it pays off. Buttons, my first Quaker was a rescue who had been abused and it took me a year to tame her down enough to keep my skin on when she was around me. I still have scars...
For the demon possession, I think Quakers just go through that but I recommend red palm oil to anyone who has a plucker. My vet recommended it by the way so I have an avian vet's thumb up on that one too.

Maybe see what your vet thinks about the plucking? Has your bird seen a vet for that yet?