I would see if anyone can pin down the diagnosis a little better. If
it's FIP, that's different from FeLV, and the progression of the disease
is not the same. If it's really FIP, then the internal fluid buildup
may signify the "wet" form of the disease. Still, I am a little puzzled
because it is notoriously difficult to make the diagnosis of FIP from
simply examining the cat or doing bloodwork. It took hard work to
diagnose my Berenice when she had it, and the vet said that it was one
of the misdiagnosed feline diseases he'd ever heard of. Also, it's not
unusual for a cat to have both. It's like a double whammy to the immune
system. In some kitties, the FIP virus lies dormant for sometimes years
and then becomes active when the immune system is compromised, which is
what FeLV does, I believe.
The Vit C has been used by many people who swear that it helps both cats
with FeLV and those with FIP, but it's not a cure-all. More than
anything, I'd suggest you monitor your kitty really carefully, making
sure she eats well (the best quality food you can afford, the better;
there are some good brands out there, like Sensible Choice, Authority,
Nature's Recipe, etc.; I'd also look into stocking some cans of Hill's
A/D, which is usually sold by vets and which makes forcefeeding a sick
kitty easier), and that she is not being exposed to any other bacteria
or viruses from other cats. If her immune system is compromised, she
will need all the protecting she can get.
When any of my cats have experienced breathing problems due to
respiratory stuff, I've used a cool mist humidifier to help them, and it
does a good job.
Make sure that the breathing problems aren't coming from FIP-related
fluid buildup, which is totally different from respiratory problem
congestion. Berenice's fluid buildup was found through some x-rays a
secondary (and more specialized) vet did on her.
You might also want to join a list where natural remedies,
specifically, are discussed, such as Holisticat, where members are very
open to and knowledgable about alternative methods of treating feline
illnesses. I think that list is a good resource. I know I have used it
many times.
I wish you the best with this challenge. I hope all goes well and that
your kitty feels better quickly.
Gabby and los gatos fregados (the darn cats!)