You and Sumauri Warrior have been dealt a devastating double blow. I have had cats with both conditions, but never both together.

I am surprised a cat that has been diagnosed with possible FIP (and don't we all hate the fact that there can be no definitive diagnosis) and showing signs of fluid build-up around the lungs is acting normally in every other way. By the time Fiorella showed signs of serious illness, she was in a state of general decline although she did live for months, living fairly normally, because she had the dry version. Her brother and sister had the wet version and they went downhill very rapidly indeed. Has Sumauri Warrior been diagnosed with wet or dry version? It's fluid in the abdomen, I believe, and as you will know from all your research, that indicates the wet version.

The only treatment my Fiorella received was general supportive care, and her brother and sister had already been adopted by the time they became ill so I don't know what was tried in their cases.

I found with my FeLV+ cat, Seshat, that her psychological well-being had a huge impact on her health. She lived with the virus (was born with it) for 7 years. What I'm saying here is that perhaps you can try the immune suppressive drug as long as S. Warrior is with you almost the whole time and content. Time spent in a cage at the vet can cause a cat with FeLV to lurch into full blown leukemia very suddenly. A cat can live with FeLV for years, but even the dry kind of FIP will usually take them faster, so you may want to target the worst condition.

Enjoy Sumauri Warrior for as long as you can, but be ready to act very quickly when he goes downhill from either disorder. As you know, cats can act normal even while they are really sick, so when you see a change in his condition you will know that you have to act very fast. And, be prepared to let him go. My Fiorella started to show neurological signs in her back end one evening, slid off the bed in the night and, when I found her underneath, had obviously gone blind which distressed her a great deal. The next morning she was euthanized even though she was still eating and enjoying attention. I just didn't want her to go through the later stages.

And, all this, of course, is simply one opinion. But sometimes other people's opinions can help you formulate a plan, can help to sort things out in your mind.

Post a picture of your warrior if you can. I'm sure everyone here would like to meet him.