Oh, how frustrating and scary for you! I get so angry at some vets. No definitive diagnosis, and she says, "Well, just put her down." How stupid and insensitive is that!!! If this idiot were a human doctor, I suppose she would tell her patients to commit suicide when she didn't know what they had?
Dr. Gonto will be of tremendous help to you. He tends to get angry (even angrier than I do, or at least he's more expressive about it!) at some of the foolish behavior of some vets, but he ALWAYS has the dog's best interest at heart. And he knows more than anyone about Fanconi, including many of the 'experts' in animal kidney disease.
Hugs to you and Stella, and thinking positive thoughts for you both!