A quick and heartfet thank-you to everyone who wrote to me privately and on this board. It turns out that our puppy has moderate hyrocephalus , some brain atrophy in the temporal lobes and spinal malformation but no synrix, thus the diagnosis of SM is on "hold". Our vet feels this falls under the SM umbrella, but since ruling SM in or out is a matter of ongoing changes in opinions, and the results are sadly the same in the case of moderate hydrocephalus in a pup this young, I suppose it doesn't matter what we call it.
Thankfully, medication works really well for her. Her quality of life has improved vastly and she's out of the miserable pain she experienced. Appetite is good too - a first in many months. No scracthing, screaming or cying - just a happy, playful pup.
Thanks again for the help you've given me at a difficult time. Sending many belly-rubs to your Cavaliers,