I'm sorry about all of this stress. Cats and humans have many medical problems, and it can get stressful if both are happening at the same time.
I will say that the wait for test results, when everything you are told with this disease is that those same results might well be inconclusive, coupled with the thought of "am I putting him through this , for an inevitable ending, and only increasing his suffering?" is what made it hardest for me.
After losing my mom to lung cancer just a year and a half ago I was in no rush to go through that again.....
and that recent history makes a non-FIP outcome that much sweeter.