I just now saw this thread. I am so sorry for your loss; it is never easy. No doubt you loved Fister very much and he loved you back. Try not to second guess your decision to stop his discomfort. I'm not a medical person but I read this entire thread and saw his lab results. Even though the results were not in English, I could identify many of them and see how high and low many were. You did the right thing--the hard thing but it was right for Fister.
I had to make the decision in August 2012 to have my Luke cat put to sleep. I feel that I waited too long because I didnt want to let him go. Perhaps most of us second guess ourselves. I do understand your heartbreak and your missing him. My condolences to you.