Hello,
Apologies if this is a ridiculous suggestion (I'm not experienced on this issue, but wanted to reply to you as no-one else has yet) but could sneezing and weepy eyes really account for stress? Is it possible your cat has allergies? Hmm, I guess the vet would have already covered that?
Your cat doesn't sound very well.
If you have tried everything else, then what have you got to loose but to try the medication that your vet has prescribed? He/she may not be a bengal expert, but as a vet he/she's still a highly qualified practitioner who has likely seen many cases of animal stress before, and his/her experience has led them to believe this medication may help your pet.
If not, how about you try to get in touch with an animal behaviourist who will no doubt cost the earth, but could maybe help assess your situation and give you the tools/information you need to turn this situation around, maybe without the help of drugs?
I hope things improve for you and your cat xxx