I don't know if you want to euthanize or just let her pass but I know that plain old aspirin is chicken safe as long as you heavily dilute it a to a gallon water, I kept one of my favorite cockerals on that to keep him eating and drinking after he broke his leg terribly and needed to wait a week...
How was the vets? Also mites prefer under the wings, along the knees and along sides at base of feathers, they also leave debris so you would probably notice uncommon dirtiness and they'd be visible in evening when they wake up to feed.
Have you checked her for mites and lice? A heavy infestation can cause lethargy, lameness, weight loss and death, heavy wings could be general exhaustian and anemia . Unfortunately the symptoms you describe don't equal a good chance at recovery but it may be possible if you find whats wrong