@Fargo No matter what happens nothing is too hard to go through, we're all here behind you every step of the way to try to help your cats.
Can you provide any information on what the vets exactly said about the cat(s) issues? I'm no animal expert but I'm sure if we heard more to the story from a vet's POV aswell we may be able to crack this case and fix the problem. Strangely enough, one of my cats will not eat tuna-- she only likes wet salmon in her food so I can understand to an extent. Technically shes not my cat, shes a family members cat which I take care of when the family member is out working but none the less that's one picky cat. From what you've described it sounds like the cat may be going through a depressive state.
Believe it or not, leaving a movie, T.V. show, or radio on while you're out of the house calms most animals down as long as there's no screaming or yelling (horror stuff). Cats are also sensitive to smell just like dogs, perhaps get a cheap wall plug in air freshener with a calming scent-- you may also use incense aswell however most smells are too strong so try to go for a lighter scent out of courtesy.
Have you noticed any abnormal behavior as in pacing like a caged tiger, coughing up what she eats or just at random times more frequently than usual, or anything that would be somewhat abnormal besides the eating behavior that was recently brought up. This includes any abnormal behavior before this occurred before, sometimes there's a handful of symptoms that start with something that may seem small but could escalate into something higher.
I apologize once again for what you're going through and I also apologize for asking so many questions. I honestly would like to help further, that's all. So if there's any information that you can give us about the cat(s) beyond the eating issue rather it's abnormal behavior previously happening or currently happening along with any information both vets told you we can do our best to try and help before making any decisions here. Reading what you posted has put me in tears, I'm so sorry,
@Fargo. Just know no matter what happens we'll be here for you.
Warm regards,
-VII