Sorry about the delay in writing. I have had software problems since the latest update by medhelp.
Let me say that I am so sorry about your loss!
I am also very sorry that you have had such an aweful experience at your vets, but just as all people are not created equal, not all vet's are created equal.
When your vet said that there may be something in your house I don't believe that he was talking about it being dirty, I think he may have meant that there was some sort of toxin or allergen exposure that you may not have any control over, such as radon, or other chemical, for example, I had a recent patient who had an anaphylaxic reaction to the bleach that the owner was cleaning the bathroom with. If the owner had not been home at the time (if a cleaning person had cleaned the bathroom instead), and brought the cat immediately to me we might never have known what had caused the problem. So sometimes we are lucky. I know none of this will bring back your cat, but please know that everyone who reads this forum has losted a loved one whether a pet or a person, so we all can symphasize.
I would be furious with the vet. I have no idea the reason why your cat died. Did they give you any specifics? Why don't you ask for all the results of all the tests and bring them to another vet to "read" the results in hopes to provide you with some answers. Best to you.