There is not enough evidence to say your cavalier has SM.
Your breeders think that perhaps she doesn't,it's great that they seem so supportive and that you have a good relationship with them.
I believe there may be low cost breeder scans available in your area.Rod Russell may be able to help locate one for you.
My feelings are that your breeder should be able to help you get one of those scans done and it would also be good for their breeding program to have some information on a young bitch who had originally been destined for life as a brood bitch.
Based on my own experience with medication side effects,I'm not in favour of medicating a cavalier unless there's a very good reason.All medications have side effects and having an unaffected dog on longterm medication to me is not the right thing to do, just *in case* she has SM.
You really do need to know what underlying condition she has in order to medicate effectively and for the correct length of time.It may be SM,it could be something entirely different.
Initially a good thorough examination by your vet is a reasonable starting point,for example has she any joint problems,patella or any post spay complications or abdominal issues.
Welcome to the site.
Sharing my sofa with Holly, Ivy,Lilly and Hazy.. and never forgetting our beautiful Daisy who reached the bridge too soon.