That's is too hot. When my bearded dragon had RI(yes I know its not a snake but its a reptile) I raised her temps by a few degrees to basically the max acceptable temp for that species. So I would probably lower the temps a few degrees and keep the cool side around 83f, hot side 88f and UTH 92f. You want it warmer so that one their metabolism doesn't slow down as you got antibiotics going through their system and two the higher temps will also kick in their own immune system. Think of it like when you have a cold and you get a fever, that's your body fighting the cold.
Fortaz is fine. I used Baytril for my beardie as I didn't want to do the whole injection thing and its much easier giving oral medication to a beardie than a snake. Be aware though that RIs can be a bear to kick and might require lots of time. I'm assuming the vet took a mouth culture to determine exactly what bacteria it is and to make sure it is a bacteria?
Also I'm guessing the vet at the least used a listening device to check the snakes lungs to make sure it wasn't pneumonia? And like I said up top, he/she also took a mouth culture to have that checked at the very least? Those are two things I would definitely make sure were done. I also had a full blood panel done on my beardie but that was because she had never been to the vet so I wanted her overall condition assessed too but those are kind of expensive.