Did the veterinarian not give you any definitive information? Since this all transpired in a relatively short period of time, I think I might write down everything that led up to & during each vet visit, what you were told and things that went on at home, then I would approach the veterinarian and at least demand a refund of all charges...and...if the vet refused, I would pursue it legally.....
Although I don't know if veterinarians carry error & omissions insurance, but I would contend that my bird was not properly examined & diagnosed, though, like Dinosrawr is right a necropsy would have been the best idea, you still have leverage in getting some recompense.....
Good luck.....