I'm so sorry your little one is having so much trouble - how incredibly scary for you both
! I'd be so upset over all of this if it were me.
Honestly, I really would seek another vet and/or an Internist to better manage the diabetes. There is something odd going on here that isn't allowing her to keep her levels stable and it's not making sense to me. Do you have a specialist vet hospital near you?
Also, the vet is not making sense saying "w/d is the only food you can feed" - when in the next breath he's putting her on i/d - lol! Two totally different foods!
Have you ever tried sprinkling Parmesan Cheese on her food, bc that can work great and adds barely anything of significance to their food...