ohhhhhh, poor kitty. Sorry you are having to go through this. Sounds like you have great vets, and it sounds like you have given your cat a wonderful life, attested to in its old age.
My only experience with this issue was with a neighbors cat who had life long issues with heart disease and passed due to heart failure.

I've had dogs and cats with cardio disease, some who were born with it and never lived to be a year, others who developed it over time, into old age.

Here is my experience with my dog Samantha who had her disease diagnosed at the age of 14. I noticed she was throwing up clear liquid and that she wasn't herself. I brought her to the vet and the xray confirmed she was full of fluid from heart disease. They gave her meds but her prognosis was not good and she only lasted about a month after. One Saturday morning she just couldn't catch her breath. The vet had told me when this happened it meant the meds were no longer going to work and it would be time to relieve her of her suffering. So I immediately scooped her up and rushed her to the vet to say good bye.

I had a Buddy also, a 19 year old chih who I rescued after Katrina. His former owner could not keep him after he lost everything and his new landlord would not allow pets. At 19 Buddy was absolutely healthy as could be then after 3 years his old heart started to fail him. I treated him for about 2 months or so and then the day came when the meds were no longer working and I had to also bring him to be put to sleep.

I had a cat 3 year old diagnosed with cardio myopathy (?spell) and no threatment would help her and 1 week later I had to put her to sleep.

Until you get an accurate diagnosis it's difficult to say how and if he can be treated. Just remain positive until you have reason to be otherwise.

The best way to diagnose a heart condition is with an ultra sound. Given your cat's age your vet will have to determine if he's strong enough to be sedated to have the test.

The reality is they may be able to prolong his life, but for how long will be anyone's guess. Don't jump to conclusions until you get some definitive answers and cross each bridge as it comes. Try to remain upbeat and cheerful around your Buddy as they can sense what we are feeling and will react to it. In other words don't let him know you are worried or else he will be too. LOL Please keep us posted. God bless.