Now you have noted that you posted the photos, but I cannot see that you have. If you can try again that would be grand.
Still, at this stage and since I do feel like we are playing a bit of tag, I will give you my thoughts thus far about your lass. Now I have to say that Chevelle’s situation does sound quite complicated. Even though I have not seen her skin, it sounds like this may appear normal (safe for the self trauma she is causing). Therefore, while she is scratching the skin and considering the tests/treatments that have been used, I would question whether our issue is actually deeper.
Now first, to hear that she has protein loss in her urine as well as this itchiness, I would note that we can see both signs on occasion in dog with kidney dysfunction. Now I assume this is less likely for her since she has had bloods (which were hopefully normal) tested and a skin biopsy done (so hopefully they’d have seen any calcium deposits secondary to uremia if present).
Furthermore, if the cause of her protein loss was stressing her body (as it likely is since she is essentially losing nutrition via her kidneys), we can see demodex mites cause chronic persistent itchiness in older dogs too. So, while again I would hope that this had been checked for, ruled out, and/or addressed; it could fit with what you are reporting for her.
And again another concern and one I hope they have ruled out already would be Alabama Rot. Now the likelihood of this would depend on your location but I would note this as a concern since it does cause skin disease as well as protein loss in the urine.
Otherwise, we have to start thinking about less common issues. Now I know that her vets have xrayed to check for visible issues within her throat and neck, but I would be suspicious that her scratching could be a reaction to irritation/inflammation with a local nerve, the spine, or even her brain. And while some of inflammations within these tissues would have responded to the steroids, it would depend on just what was present. So, I would consider potentially speaking to he vet about these nervous tissue irritation causing what you are seeing. If there is a question of this, there are medications that could be trialed (ie Gabapentin) to rule out nerve pain. As well, you could even consider having a spinal tap or MRI to check for diffuse infections or protozoal disease that could affect the brain or spine.
Overall, you have taken all the right steps so far. Still, this sounds like this may be something subtler then a skin issue here. So, if you can post those photos that would be great but otherwise these would be other concerns for what we are seeing with Chevelle.
Please take care,