Instead of scratch, I'd offer a "just past damp" mash using her balanced feed. Get it wet enough that it just sticks together. Start with a small amount.
Use a permethrin based product. DE won't get rid of them. If she has them, it's likely all the others do too. All birds and housing will need to be treated.
I like using Gordon's. Here's a link to dilutions.
Post in thread 'Depluming 😭'...
Fluids and nutrition go by weight or target weight. Please take a look at the thread I linked.
Here is a quick cheat sheet for volumes:
Here is a thread where a few of BYC's experienced tube feeders taught someone how to do it for the first time.
Thread 'Need to Tube Severely Molting Hen'...
@casportpony has a great tube feeding article here:
https://www.backyardchickens.com/threads/step-by-step-tube-feeding-guide-pictures-under-construction.1064392/