Hi and welcome to the board; I'm so sorry that you are having this perplexing problem and cannot find any clear solutions. Gabapentin helping would I'd think suggest possibly some kind of neuropathic pain/nerve pain, unless it is perhaps making him feel dozy and less inclined to notice whatever is bothering him. I am assuming the full body X-rays removed hip dysplasia as a possibility? Cavaliers have a higher rate of this condition than many breeds even though it is often thought of as a large breed issue. I'd wonder too if this is something spinal that isn't being picked up, a compressed nerve somewhere? I'm not a vet and am only offering some thoughts based on having had numerous cavaliers with various degrees of SM. CM will show in nearly every single cavalier on MRI as I am sure your neurologist likely noted to you. Keep in mind much depends on the abilities of the radiologist reviewing scans and details like the angles of the scans and positioning of the dog. As with human patients, different radiologists can see different things depending on experience and ability and areas of expertise. It does sound like SM, or CM as the cause, has been ruled out although I am not sure that all neurologists would agree that fluid buildup needs to be present to cause pain with CM; I know that fluid dynamics can be a key issue -- ie the way the cerebrospinal fluid is moving about the spine and brain.
Perhaps it would be useful to get a second opinion on the existing scans? I am assuming that some simpler possible causes of yelping and snapping at hindquarters have been ruled out, such as flea allergies or rabbit mites (Cheyletiellia)? I've known of a few cases where people had dogs snapping at hindquarters and feared SM but then found it was rabbit mites, which can be extremely irritating to dogs. Some vets don't seem that familiar with them because they affect dogs less frequently than rabbits and cats, but two of my cavaliers had these as puppies.
Neurologist Clare Rusbridge says this about alternative diagnoses (
https://veterinary-neurologist.co.uk/cm-p-and-sm-s-clinical-signs/):
I would think a dog having signs of CM would likely be showing other signs than snapping at hindquarters. These are the signs Clare lists on the same document: