Hi, I'm back from the world of no internet, sorry it took so long! Not quite, the trout fishing was great, brown trout and flies and a small boat is a great combination.
I'm sorry to hear your troubles are continuing, and I'm worried about your vet (referral vet)'s attitude to the problems. If they don't get results from their tests they are doing the wrong ones and as I have mentioned earlier: a complete physical examination is where it starts! It is so easy to forget in the world of quick'n'easy testing, but the first, most important and best test is the physical. Then you treat what needs immediate treatment, and dehydration is top of the list there. No argument can sway that one, dehydration will kill any feline given time. Low blood pressure: prime suspect is dehydration, best cure is rehydration. Blood loss: rehydrate. diarrhoea? rehydrate ( I once rehydrated a small shi-tsu puppy with parvovirus enteritis, it weighed 600 grammes at reception, and got the same volume Ringer-acetate iv in 36 hrs. Never seen an owner cheer so much at being bitten by his puppy, no blood though!) . If you can't find the cause of dehydration, at least make sure you give enough fluids, food is secondary and may increase problems if the cause is intestinal. In the case of unknown causal agent, shotgun (broad-spectrum) antibiotic and antparasitic drug use is smart, and also vitamin B, especially B6 and 12. By the way, have you got any crimidine-based rodenticide lying around, or fed any raw pollock?
And if your vet doesn't listen to you, find another!