I am so sorry you have been through such a terrible health crisis and have suffered the loss of your beloved son. I cannot begin to imagine your pain. My deepest condolences to you and your family.
I am also so sorry you are facing what looks like an SM diagnosis with your cavalier. We have a lot of information here on the site and I'd also recommend checking the information on
www.cavaliermatters.org which is very pet-owner friendly. Dr Clare Rusbridge, the leading researcher on this health issue for over two decades, also has a website full of information that will be helpful to your vet and to you. She also has a YouTube channel that explains the issue and treatment options.
I've had several cavaliers with varying degrees of CM/SM. Some only had very mild symptoms when elderly while my most affected dog was MRId solely for research at around 18 months with no symptoms and turned out he had CM/SM and developed symptoms when around 2.5. He was easily managed all his life til age 10 with meds and died from the other major breed health issue, mitral valve (heart) disease. Far more owners will lose dogs to MVD than CM/SM. In most cases, dogs can be managed on meds and it's best to have this done by a neurologist who can also work with your vet. Often CM/SM dogs need changes to meds over time and often need a mix of meds, too. Some dogs are good candidates or better candidates for surgery, which can halt progression in many cases but often dogs do still require meds. I know Dr Rusbridge generally advises that surgery is a good option for dogs that have early onset CM/SM or later onset that eventually cannot be managed adequately by meds. The surgery is generally quite safe for dogs. That said: there are dogs who cannot be well managed and who end up with quite severe CM/SM and it's kinder to let them go. But these are a small proportion of affected dogs overall -- in my experience of talking to many many owners and to neurologists around the world over the past 20+ years. Also: researchers now believe many CM/SM dogs develop symptoms but then stabilise at some point and don't see any progression after middle age. So it's hard to give any definitive answers or predictions as each case is unique.
Yes, a dog can be diagnosed based on symptoms and Dr Rusbridge has links on her website to a paper that suggests this as an option when an MRI isn't affordable or TBH, perhaps needed anyway. Unless someone is considering surgery, I personally don't think there's any need for an MRI as symptoms are fairly obvious and air scratching in particular is not seen with other diseases. And yes you can have vast improvement on the right meds. Dosage of gabapentin is based on weight and often needs to be given 3x rather than 2x (not always). Usually once you give it you should see improvement in symptoms within days or a couple of weeks (the trial period is usually 2-3 weeks). Your neurologist will be able to advise once you have your visit and adjust dosage and frequency as needed, and also there are other meds that can be added in or if gabapentin isn't very effective, pregabalin, a newer related medication, often does the job. Have you seen any improvement since you started the gabapentin?
As noted above over 50% of cavaliers are likely to eventually develop SM and almost all, probably 99%, have CM, the skull malformation that is believed to contribute to the development of SM. It's also identified in many other toy breeds with flatter or flat faces, as well as Staffordshire terriers. However, I have had a house full of cavaliers since around 2003 and of those only a few have needed meds for CM/SM and been symptomatic in any way. I currently have 4 cavaliers and a cross and none show any symptoms; they range from age 1 to 9. I would not be without a cavalier. It takes time though to find a testing breeder. A good breeder should share testing results on parents before puppy purchases and go through her or his choices in a breeding programme. There are several tests that should be done on parents, and these are outlined on Cavalier Matters. Unfortunately the clubs internationally (excepting Scandinavia and the Netherlands, and more recently Ireland) haven't shown much real interest in addressing the breed health issues nor have the national kennel clubs. There's a lot of denial and 'breeders know best'. Looking at annual puppy registrations through national kennel clubs, though, it's easy to see how many prominent breeders breed younger dogs (long before symptoms or tests might show problems) and don;t MRI or sometimes even properly heart test despite many of these things being subsidised through clubs.
So: yes, the condition is common in cavaliers and will often show on MRI even of asymptomatic dogs, but only a proportion are symptomatic and a smaller proportion, serious enough to need lots of meds or surgery and more rarely, end up being euthanised because the condition cannot be managed. There are dedicated breeders who are testing properly, and following breeding protocols, which basically mean doing MRIs on breeding dogs only after age 2.5 and breeding according to research recommendations. It can be very hard to find such breeders. I would very much differ on the point of breeding being a 'crap shoot' however: there are now literally years of research studies showing unscanned dogs bred to unscanned dogs especially at younger ages greatly increases the chances of CM/SM and of more severe cases in offspring, whereas scanned dogs with good grades bred under the recommended protocols end up with 25% or fewer affected and those tend to be less symptomatic. Many show breeders -- and almost no non-show breeders -- scan though and they still often produce the same old tired lies about how testing is unreliable and it doesn't matter and their own lines are unaffected (which they can't tell if they don't scan). There are effectively no unaffected lines (just as with heart disease). Sadly some of the lines now known to have been genetically less affected have disappeared over time due to breeders focusing on breeding from show-winning dogs (producing 'popular sire syndrome' genetic bottlenecks).
I'll add some links for you to follow. Please let us know how your appointment goes and ask all the questions you want. And of course you are very welcome to vent all you want, I hope it helps in some small way to just get some of these things down in writing. Wishing you all the best for your appointment and sending my heartfelt sympathy for all you have been through.
-- I'd try the routine the neuro-vet prescribed for two weeks. If no real improvement to scratching etc I'd ask about increasing gabapentin to 3x. If no real improvement I'd ask about switching to pregabalin. Then at that point (or with gabapentin) there are many pain relievers that can be tried as per Clare Rusbridge's treatment algorithm. Note that they are somewhat different for CM and SM so you'd want the neuro-vet to have some idea of which to try.