If the itching and biting is still there then I would start the pain treatment. I am sorry but nowadays, as you own vet has told you, there has to be a very good chances that this is SM.
Dogs cannot say they are hurting, we can only try and 'hear' them by interpreting the symptoms they display. Try the painkillers, see if it makes a difference to her level of comfort
Cavaliers......Faith, The Ginger Tank and Woody.
Japanese Chins.... Dandy, Benny, Bridgette and Hana.
Remembered with love......... Tommy Tuppence and Fonzi