My experience with Wobbler's is similiar to bubba's - in young horses with such poor neck conformation that there is nerve compression somewhere in the neck that creates the neurological symptoms.
I've never heard of it before, but I guees it's possible that gray horse melanoma could cause something similiar. The other difficulty is there's no hard diagnostic criteria for Wobbler's, short of autopsy - the diagnosis is made by observing the animal's confo, movement and behavior.
My first question is, was a workup done for EPM? EPM and Wobbler's look a lot alike in early stages, and it's just as likely, if not more likely, that an EPM lesion is causing the symptoms than a gray horse melanoma. The good new there is that the treatment for EPM is non-invasive. (Can be expensive, but it's not surgery.)
Frankly, Wallaby, I'd get another vet out there ASAP. Without a definitive diagnosis, you're not going to be able to enjoy your horse, and you're going to wonder if you're doing the right thing every time you get on her. I know she's older, and you want to be enjoying your time with her. Imagine how you'll feel if it's something else entirely, and treatable.
Get a second opinion. Immediately.