There's a research on cats' water intake. Basically saying that cats need to take water from food. Even if a cat is drinking a lot of water, it's still not enough.
Alas she doesn't take anything but dry food, I even tried cooking chicken etc. I had some success recently with a particular brand of wet food and their tuna flavor, I don't remember the name but it was one that did not contain chicken byproduct, which is why I bought it. She drinks a lot though, usually empties her water bowl in one or two days, so I don't believe its dehydration. The other vet gave me cortisone pills to be used after the injection wears off, I might try that, especially at present with the current situation. It may help her get through this wound phase with less itching. I try the egg thing, I never heard of that before, but it's worth a try.
http://www.catinfo.org/docs/WalthamDietaryWater.pdf
When my cats were eating dry food, they drunk a lot of water. Now I'm feeding all wet, they don't drink from water bowl anymore, but they pee a lot more, so they are definitely getting more water with wet food.
Have you tried freeze dried raw? My cats go crazy with Stella & Chewy. Maybe you can give small amount as treat and slowly add some water?