I can understand your mixed feelings, as both of mine have CM/SM. I don't think there's anything like 'average progression' - both for symptoms and progression it seems to be a very individual disease. My Oliver was diagnosed at 6, but had probably had it for several years before; he's now 12 and isn't really all that much worse, especially since we put him on a different drug two years ago. Aled is 6 now, was diagnosed at 4 and is already on a higher dose of gabapentin than Oliver, but has another chronic disease that really causes him more daily hassle than his CM/SM. On the other hand, there are many on this forum who are struggling to control severe pain in their SM dogs - and sometimes failing. It does seem that the younger the diagnosis, the worse are likely to be the symptoms, but apart from that it's just constant watchfulness and frequent tweaking of medication and dosage to keep them as comfortable as possible. All I would say is that it is important to let them live as normal a life as possible and don't mollycoddle them. Walks are often an opportunity for them to 'switch off' and forget about aches and pains, and all Cavaliers seem to have a capacity for enjoying life to the full however much that life is curtailed by illness. CM/SM is a lottery - unfortunately you don't have the chance to win a million pounds!
Kate, Oliver and Aled
PS What dosage of gabapentin is Nelli on? The usual starting point is 100mg twice a day, but in fact most dogs seem to do better on three times a day (and Oliver is more comfortable on 4 times a day) - dogs differ in how long the drug takes to work through the body and it's important to keep the gabapentin level up, so that there is 'blanket coverage' of the pain. Pain level can also be affected by weather and air pressure, so if your vet agrees, you may need to give an extra gabapentin occasionally - and become an expert air-pressure-watcher, like a lot of us!