I'm sorry to hear about your parrot! I have an African Grey that has plucked for years.
In her case it's because she came from a bad breeder who never bothered to socialize her and then she spent the next 4 years in a tiny cage in a pet shop. She plucks like people nervously bite their nails. I've tried everything but she's determined!
But I worked at a pet shop for 9 years and we had an Amazon that would pluck in the winter and stop late spring. He would just let his feathers grow back like nothing happened. I think it was the dry winter air myself but who knows?
Sorry that i don't have much advice since I'm in a worse situation but I thought that might give you hope!
Have you ever tried pellets for your parrot? They are much better for them then seed and my Baby LOVES them. There are some people that think that certain seeds when eaten too much can trigger plucking. I have no idea if it's true but it can't hurt to try it sometime.