She did fine on the bland diet. At the beginning of the seven-day transition period, we left to spend three days in Los Angeles for Susan's sister Kathy's wedding, but one of Susan's co-workers came over twice a day to feed the dogs and began transitioning Boston toward regular food. As far as we know, Boston was fine throughout the time we were gone. When we came home, she was still fine, although we did notice that she mostly ate the rice, cottage cheese, and chicken portions of her meals, and largely avoided eating the regular dog food mixed in with it. Yesterday, we tried transitioning her back to mostly regular food. She was fine for about four hours after eating, but then she started throwing up. Whereas a week ago she was throwing up horrible stinky stuff, this time she was throwing up almost pure water, and more frequently than before. Approximately every ten minutes, she threw up a big puddle of clear water; then she would spend much of the next ten minutes gulping down more water, only to throw it all up again. She also seemed (understandably) lethargic throughout this - until all at once, she zoomed across the room as if something was chasing her, spun around, staggered backward, and started rolling around on the floor and twitching. At that point we took her to the vet.
The vet ran some blood tests and said the weirdest result was that even though she was dehydrated both tonight and last week, her urine was unusually dilute both times. She was even more dehydrated this time, yet her urine was even more dilute. Also, she had abnormally low potassium levels a week ago but abnormally high potassium tonight, which was strange. As for the weird staggering and twitching this time, the vet said she seemed to be having partial seizures, which could indicate poisoning (except that we're pretty sure there's nothing she could have gotten into that would have poisoned her) or that she's developing epilepsy (except that it would be strange for her to inexplicably develop epilepsy at approximately 5 years old, and there would still have to be something else that caused the vomiting) or that her electrolyte imbalance was just thrown way off due to severe dehydration (which seems to be the simplest explanation).
She is supposed to have an ultrasound today that might help narrow down possible diagnoses. For now, it seems like the most likely diagnosis is Addison's disease, and the second most likely is diabetes insipidus. Both would be permanent conditions that she would need to be treated for for the rest of her life. And we have already spent close to $3,000 on these two trips to the veterinary hospital - Susan is offering to pay most of it, and I think I will accept the offer, but it still makes me uncomfortable to be spending money that we can't spare, when I have savings but no income and Susan has income but no savings, and this is just adding to her debt.
Edited to add: The ultrasound results are back! Her adrenal glands are normal, which makes Addison's disease seem less likely (although we're still waiting for the results of a blood test for Addison's disease). There is a thin spot in her stomach lining, which could indicate an ulcer, so I guess that's the most likely diagnosis now (although it could also be a thin spot just from throwing up so much). The treatment for an ulcer would apparently be to keep her on a bland diet indefinitely and retest her periodically to see if the ulcer goes away. They're keeping her hospitalized until tomorrow, to watch for more symptoms and hopefully get a firmer diagnosis.