Toxoplasmosis, Cryptococcis, Bartonella, Feline infectious peritonitis, feline immunodeficiency, and feline leukemia virus often present as anterior and posterior uveitis, chronic uveitis, retinal detachment, and secondary glaucoma, which just means that systemic diseases can result in ocular disease such as sudden onset of blindness. Your cat did test neg for feline leuk. but your cat should be tested for these other common diseases. Feline infectious peritonitis, and Feline immunodeficiency are viral infection have no cures but can helped by boosting the immune system. Toxoplasmosis, Cryptococcis, and Bartonella can be treated with the appropriate medications.
This sounds like it might be distemper. I've read symptoms of this, and your kitten fits some of them. I sure wish I knew what the treatment is for this. Did your vet check for this?
I'm also wondering what antibiotics your kitten was put on-was it baytril? I have heard of baytril causing blindness.
I really wish you and your kitten best wishes for a good outcome. I know this must be incredibly stressful for you, so take care of yourself, too!