Steve,
I know you are having a hard time believing she is diabetic but if everything else has been ruled out and she still has these kind of numbers then she is diabetic..She might not remain insulin dependent forever, and she might need a very small microdose of insulin for a short time to get her into remission. But the longer she stays in these numbers the hard it is going to be to get her into remission later.
If you aren't giving insulin yet, and your vet remains adamant about not putting her on it, please at least start checking her for ketones. Not eating and lethargic is a cause to be concerned. Things that if she was mine that I would be asking the vet to check for is infection...UTIs can cause them to behave the way you are describing, as well as pancreatitis. And so can DKA...since the recipe for DKA is not enough insulin, not enough food and infection. I'm very worried about her as I am sure you are, something is just not right here. Also not eating in cats can lead to Fatty Liver which can be life threating. If you can please try to get her to a vet today, I even think if she was mine I might be taking her to a different vet for a second opinion.
Mel, Maxwell, Autumn & The Fur Gang