Welcome to our forum. Know that I am not a vet tech and have no medical education to help you out. You have given a ton of information and it really seems ODD that FOUR different vets cannot determine what is wrong, especially after doing scans. I honestly do not know the difference between a CAT scan or an ultrasound and if they are different, maybe the other one should be done.
I don't know if you have Googled anything, but here is an article that I found which seems to imitate what the vets have been saying:
https://pethelpful.com/cats/Possible-Is ... n-or-BellyYour gal is acting normal, however, she is a senior kitty, and I understand that surgery can be dangerous for her. I would ask the vets to use any non-invasive measure (X-rays, scans, ultrasounds, etc.) and see if they can find anything. Certainly, there is SOMETHING there. And they should be spotting something, especially if the abdomen is swollen.
I know this concerns you to no end and all I can advise is to continue to search for answers through veterinarians. They have the expertise that we do not. I know you want answers. Definitive answers and I would NEVER take an "I'm not really sure," attitude from a vet. There are vets who specialize in different areas of the body. You may want to get a recommendation from one of your vets.