Firstly, I am sorry I am just seeing this now. Sending lots of prayers and good healing thoughts to Chestnut. A big hug for you my dear. So much to have to deal with.

Secondly, it's a good thing your vet is a ways away from me or I'd go give her a good head slap! Did they take precautions when dealing with a cat that isn't feeling well? He's up to date on his shots so there should be no issue. Isn't that why we vaccinate our pets, so there are no issues if a scratch/bite happens?

Perhaps now that they've come back with really no results it is time to seek a second opinion.