I'm not at all surprised that she knew nothing of SM or PSOM. Not sure if anyone remembers, but when I attended the AVMA convention in Atlanta last summer (the main source of getting continuing education credits for practicing GP vets) I attended all of the seminars on SM, and in each there was only a handful of attendees - 8-14 or so vets. But in the seminars on liver disease, kidney disease, heart disease, there were hundreds of vets crowding the room. The SM specialists who were presenting all commented on how disappointing the attendance was at their sessions.
My own GP vet would not know of these conditions were I not her client. She has always been open to my bringing in material (from valid sources) about particular problems common to Cavaliers. One of her colleagues actually once phoned me to come in and do an "observation" on a Cavalier (the owner was not present). But many vets don't have an open attitude about input from clients. The best approach is to bring in printed material from a valid veterinary source (like papers from Clare or Shores or Dewey) and ask them to look the material over when they can.
I would expect that most vets would not know about proton pump inhibitors being used as CSF reducers because this is an off label use of the drug. One would only know that this drug is used for SM if they had read up on the condition, diagnosis and treatment.
http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0000936/
A thought about Blondie's coughing - has any vet asked to do chest x-rays to check for primary respiratory disease (upper or lower airway)? Have you discussed the possibility that she is particularly sensitive to smog, pollution, etc since she is a "city" dog? We're having a lot of "bad air" smog days in the big cities where the weather folks say that sensitive groups should avoid going out.
Pat