I would be very angry, as well, that this vet wasn't taking you seriously. You know your dog better than anyone. I am glad it will all be resolved.
I did want to point out one thing, though, in case it helps a dog suffering with SM pain. MRIs are very expensive, especially here in the US (here in Atlanta it is around $2,000). When I first got Scarlett she was about a year and a half-- her breeder had leukemia and had to place her. I offered to take her in. As time went by, I started to notice some SM-like symptoms (air scratching, limping with her back leg- always the same side, head rubbing). At that point in time, I simply couldn't afford the $2,000 to MRI her but I had done a lot of research and knew what I was seeing. I talked with my vet and we decided to try her on the Gabapentin and see if we saw improvements, as typically, Gabapentin helps SM. As soon as I started her on the meds, things dramatically improved. I did take her in to see the neurologist a few months later for a consult and ohysical exam and brought video with me. The neuro agreed it was most likely SM and added omeprazole to her meds. All of this made a huge difference in her quality of life.
I have since had her MRId and she has a horrible case of SM. Even though I couldn't afford the MRI at first, if I hadn't started her on SM meds, she would have suffered quite a bit. So, sometimes, if you work closely with your vet that you trust, you can try the SM meds to see if they help the symptoms. Of course, an MRI is always the best route, but it isn't always financially possible.
Holly, Oliver, Rosalita, and Scarlett