The most commonly used remedy for scale mites seems to be plain old Vaseline rubbed on the legs. It smothers the mites rather than adding more poison to the birds' systems. Try to keep her hydrated, you might try to wet her food and make it into a mash - in very small portions so you can monitor her intake. The moistened food is more palatable and easier for compromised systems to handle, plus it adds another level of hydration. Hydration, warmth and quiet will be your best bet at this point.
Just a note, and NOT a criticism....always, always be sure you understand the vet's instructions fully. Ask, ask and ask again for clarification until you have it down, or ask for all instructions to be in writing. I think you and the vet were reading the same book, you were just on different pages. I remember once we had a dog in our practice that suffered from repeated bacterial and fungal infections in her ears. We tried her on a new antifungal oral med with some 2xdaily ear wash and dryer agent for her latest bout. Her owner called a few days later to tell us that the medication was making her dog ill with some vomiting. The vet advised her to add a little Pepto Bismol to the treatment - not a lot, just about a 1/4 tsp, and bring the dog back in two days. She did, and when I looked into the ear canal I couldn't believe the mess I was seeing. Turns out instead of giving the Pepto orally she was putting 1/4 tsp in her ears after putting in the ear wash and drying agent. No one person messed up here - everyone messed up! She was at fault for not understanding and questioning the doctor's instructions, and he was at fault for not being specific with a panicked and concerned owner. Same book - how to treat the chronic ear issues. Different pages - no continuity. A missed communication that added to the misery of an already sick pet.
Please, keep us posted on how she does. And relax - not one of us out here have raised chickens and not made some mistakes along the way! We're in your corner and want the best for your little banty.