I'm so sorry to hear about the problems you are having with Jupp. I know how worried you must be. If you don't feel confident about what your vet is saying, maybe you could ask either for a referral to a specialist or for a second opinion from a specialist greyhound vet? I'm always asking people if they know of any good greyhound vets, just in case one day I might need a second opinion from someone who knows more about greyhounds than my own vet does. I'm not sure whereabouts you are but I have compiled a short list of recommended vets in Hertfordshire, Essex and Middlesex if this would be of any use. From what I understand, it's quite possible that you might end up paying a lot of money for expensive tests that may or may not bring you any nearer to a diagnosis and vets are probably used to dealing with people who can't afford or who just don't want to spend that sort of money. You just need to make sure that your vet knows you're willing to do whatever it takes to get Jupp better.
Will be thinking of you today as you await the results of the x-ray and checking back for updates.
Good luck