I just read through this entire thread, and the problem I'm seeing here is that even to-date now, this Goffin has not ever been taken to a CAV or Avian Specialist for blood-work or any Cultures! It's impossible to start successfully treating a self-mutilator without first ruling-out a physical cause, such as disease, illness, allergy, nutritional deficiency, etc. Instead, you're just guessing and spending more money than the CAV visit and the testing would take. The OP wrote that the bird will not be going to the vet until October, and I don't know if they even plan on having blood-work and at least a fecal culture done at this upcoming appointment or not. I hope so, as unfortunately stopping this may be as simple as reading the blood-work results or finding a horrible fungal infection throughout the GI Tract.
The other issue is that the OP never mentioned what brand/type of food the bird is eating, but did mention that it contained both "corn and pellets in the food", which to me indicates a cheap, bagged seed-mix that contains cheap, colored pellets and pieces of corn, as the bags of parrot food at Walmart and grocery stores have. It's obvious they are not feeding simply a natural-pellet as the staple and then a low-fat, high-quality seed-mix that contains no sunflower seeds, no cheap pellets, no nuts, no pieces of corn, etc. So this could be causing an allergy, a nutritional deficiency, or both. Instead of buying a high-quality, natural pellet they are trying vitamin powders and pet-store feather deterrents, and these are typically not the answer.
The bottom-line here is that the bird needs to get to a CAV with feather-destructive behavior experience, needs to have a full blood work-up and at least a Fecal Culture done, and then go from there. This could be something as simple as a nutritional deficiency due to the poor diet, an allergy to the poor diet or an environmental factor, or it could be something as serious as a liver or kidney disease...no way to know without the bloods and the cultures.
What are the 8 most common allergies?
