Any possibilities I can think of, it already sounds like both vets have considered: collapsed trachea, asthma, tumors, heartworm, etc.

Some info on feline heartworms. They are a bit different than in dogs, including the testing for them.

Feline Heartworm Society

However, heartworms do not need to develop into adults to cause significant pulmonary damage in cats, and consequences can still be very serious when cats are infected by mosquitoes carrying heartworm larvae. Newly arriving worms and the subsequent death of most of these same worms can result in acute pulmonary inflammation response and lung injury. This initial phase is often misdiagnosed as asthma or allergic bronchitis but in actuality is part of a syndrome now known as Heartworm Associated Respiratory Disease (HARD).


Not that you haven't already spent a ton of money on tests, my only other suggestion would cost probably as much, but may provide some answers. Have you tried taking your cat to a specialist, someone that deals specifically with respiratory problems? One of your vets should be able to refer you to one that may be more familiar with this sort of thing.

I can understand how helpless and anguished you must feel. It's awful to see our furry friends suffering and not know how to help them feel better. I hope your kitty gets better very soon and is with you a long time to come.