I think the problem is that this is all hypothetical- WITHOUT the additional testing done.
The problem is the kitty is sick. The only possible way to solve the problem is to get the additional testing done.
If you have the money (which, from the sounds of it, you do, you just don’t want to spend it on the additional testing) I would really, really, really, recommend giving it one more go with the additional testing. At the very least, I’m assuming the kitty is suffering to some degree right now, and it seems cruel to let him suffer if you have the means to try to fix it. I certainly couldn’t let an animal of mine suffer because I was undecided and spending days and weeks weighing the pros of cons of going back the vet.
What if you do the testing, and it solves the problem? All this stress, worry, and guilt would be gone. It seems like you and your husband are so hung up on what if it doesn’t work, but what if you get the answer and it is something treatable. Or if it is something that isn’t treatable but will get worse, and the poor kitty would have a poor quality of life, then you have that information to make the kindest decision to relieve the kitty from the suffering through euthanasia.
Wouldn’t you rather at least try, rather than wonder what if? I know if I ever brought an animal back because of an undiagnosed illness, that I had the means to at least try to get a diagnosis, I would be riddled with guilt for the rest of my life.
My personal experience- when my kitty was holding her jaw open slightly, I took her into the vet, thinking it was a minor dental issue. Vet suspected jaw cancer, and gave me the option to do a $600-$800 biopsy to confirm. I chose to go through with the biopsy, because if it was cancer, then we would know to start palliative treatment and pain meds to keep her comfortable, and if turned out not to be cancer, then they would be able to treat whatever minor thing it was.
It was cancer. I would have felt horrible if I hadn’t done the additional testing to confirm the problem, and to start “treatment.”
If you really do not have the funds for $700, then I would tell the vet how much you do have, and see what they can do.
I also would start saving up ASAP for a vet emergency fund. $500 in vet expenses is not that much. I say this as someone who makes under 20 grand a year, lives pay check to pay check generally speaking, and has spent easily 2 grand on my dog and on my past kitties over the last few years, which is one the low end for a lot of people when it comes to vet expenses.
I also want to say I can empathasize with how miserably stressed you are dealing with your sick kitty. Not sure if you know much about jaw cancer in cats, but it is very aggressive. Within a week after the diagnosis, my kitty couldn’t eat normal food anymore and was lethargic. I had to spend hours upon hours each day mixing kitten milk, chicken baby food, fish broth, wet, crumbled cat treats into a pasty mush, heating it up slightly, and follow her around everyone to try to get her to eat. She wasn’t able to groom herself well, and would get poop stuck and smear it around our home. It was so incredibly hard to care for her, my life revolved 24/7 around caring for her. I was exhausted, and I only had to do that for about 2 weeks as I put her down before she started suffering or not being able to eat, I couldn’t imagine doing it for months straight.
It is easy for people to say they would live with it, if they actually haven’t had too.
I really think the best solution to this needs to be another vet visit. I understand your husband doesn’t want to spend the money, but surely $700 is worth it to at least try to “fix” the kitty? I have a hard time imagining you don’t have $700 to spare if you are TCC. If you honestly don’t, then like I and another poster said, see what the vet can do with your budget.
This whole post iteself could possibly be for naught if you actually get the testing done to find out what the issue is.
But no, I do not think you should bring the cat back to the shelter, especially when you are keeping his sister, and because something that could solve this problem 100% completely is to go back to the vet and get the testing done.