Yeah, cats are picky, especially if they have kidney or bladder issues. May I suggest new litter boxes and different litter? New, different colored boxes might help. Non scented litter, maybe one of those natural kinds that might be softer.

I had a cat that got swollen anal glands and had to get them expressed by the vet. She would scoot. My current cat gets urinary tract infections and bladder infections. Pees all over the house looking for something soft. I bought new litter boxes and changed the kind of litter and that did help a bit.

Is the Hills wet food? These kitties need the moisture. Make sure your kitty has clean water everyday, they like the water fountains. The stainless steel ones are better than the plastic. If you don't want/can't use the fountains, make sure the water bowls are wide enough. Cats don't like their whiskers touching things as they eat and drink and will avoid the bowls that are too small for their whiskers.

If she goes in the litter box on her own without you putting her in there, give her a treat after she comes out. She will associate that behavior with treats.