I wasn't expecting this. Came here to look for an update thinking he'd be doing better. I kept sending good thoughts but sometimes that's not nearly enough.
I'm really sorry for your loss and I understand how you feel.
What a handsome brown tabby he was. Maybe now he runs in the lands of the eternal summer.
I didn't want to mention it before but I also lost my Peanut (dog) about 5 years ago to a mystery illness. His immune system was compromised yet he didn't have any of the typical diseases that could cause it. Not even cancer and he faded away. One morning he was cold, I took him to the hospital where he fought for like a week with a feeding tube in the side of his neck. They did all the tests possible and could just not tell me what he had so I never knew and I still don't know. And I still have a broken heart. So I understand you. I've had my losses.
Rest in piece Diablo you are no longer in pain little angel