Well, for starters, he was in a coma in the ICU and had to undergo several unplanned procedures and a planned surgery. It seemed consistent with some bacterial infection at that time (C. Diff?), but a year later, he still had weakness in one arm. So paranoid or not, he had objective symptoms of a very serious illness.
I read only SP’s posts in a photographer’s journal when married to EP, and the eulogy he gave to parents. In the posts, he was not devoid of some funny, albeit slightly clumsy, sense of humor when complaining of EP’s anger. She didn’t like to be taken photos of, and his phrase “the reason I am still alive is because I don’t try” indicates the tendency to joke off when the situation is difficult. I can’t see much more, but I doubt he was that paranoid before he ended up on a ventilator for almost a month. This changes one’s outlook on life. And even after that, he wasn’t suspicious of EP, but then he felt unwell again. To me, EP seems rigid, unforgetting and overall, a “heavy person”, never letting things go. SP is sort of a lightweight compared to her. (BTW, in the “death wall” I see dad’s trait in the kids - using humor to deal with an uneasy situation.)
So of course, we don’t know much, but I think his suspiciousness developed over time. Whether validated or not, we don’t know, of course, and while I am very suspicious about the mushroom poisoning, SP’s case is 50/50.