Hello, my name is***** and I have over 20 years of experience as a veterinarian. I understand your concern about not treating a disease that has been diagnosed, that does seem counter-intuitive.
In your veterinarian's defense however the side effects from drugs used to control Cushing's disease can range from unpleasant (nausea, diarrhea) to dangerous like liver damage or an Addisonian crisis (over-suppression of the adrenal gland) and very, very rarely sudden death. So my rule of thumb is that unless the dog is showing obvious signs of Cushing's disease (insatiable appetite, significant increase in water consumption, secondary muscle loss, poor skin and coat condition, secondary infections, or hypertension) then I don't treat them, I monitor them.
If her blood pressure has remained normal then I doubt she is suffering from headaches.
I can tell you both of my retired greyhounds love to sleep with their heads hanging off their beds, on the floor, lower than the rest of their body. One likes to sleep on the stair landing with his head hanging halfway off the landing. It doesn't look comfortable to me, but these boys are all about being comfortable, so I know that they wouldn't do it if it didn't feel good. Perhaps stretching out their necks feels good to them. Neither one has Cushings, or any sort of hypertension. So I think your older girl Dorrie is just making herself comfortable, and I don't think that her head position while sleeping has anything to do with her Cushing's disease.
Please let me know if you have any further questions.