This scenario sounds a little familiar... It seems that it isn't always as easy as we sometimes think it should be for veterinarians to figure out whether or not an animal (reptile or otherwise) has an infection, and if so, what kind of an infection, and then how best to treat it. :?
When our beardie was diagnosed with adenovirus two years ago, the first thing that showed up was swellings in his elbows and wrists, and resulting limping and shaking, probably because it was painful. The vet aspirated the goop and looked at it under a microscope, but couldn't see anything but "amorphous debris" -- no obvious microorganisms and no obvious uric acid crystals, etc. And they could never culture anything identifiable from the goop either, although a few attempts were made. Eventually, after several rounds of what I suppose were broad spectrum antibiotics, he just seemed to slowly recover over time, although his situation was a little different than what's happening with Shelby.
I guess the bottom line is that sometimes all you can do is keep them warm, keep them
hydrated, feed them the most nutritious food you can get them to eat, keep them as comfortable as possible, treat whatever comes up as best you can, etc etc -- in other words, "supportive care", and hope that for all that, maybe their own immune system can get a handle on it so they can recover. It sounds like you're getting a lot of good help both from your vet and from the people here, so I hope that some or all of these suggestions will help Shelby.
BTW, does anyone feel like they're up on the latest research on probiotics? The last time I checked, the jury was still out about what sorts of benefits could be reasonably expected from them, as well as what particular mix of microbes would be best for which individuals and in which situations, for humans as well as for animals. The idea that if it doesn't help, it probably won't hurt, is probably a pretty safe guess, but I'm wondering if anyone knows of any recent research. It seems that our knowledge of the "microbiome" is still in its infancy because we haven't had to worry about it until fairly recently in our history -- until we got ourselves and our pets into artificial environments, it probably pretty much took care of itself most of the time. :?