I am trying to look at the situation from two sides.
One, EP's. Now we know that she did work, several jobs, and is a smart person. She was unhappy that SP was always absent. It seems that their lives were on divergent courses for quite a while. If, as her friend says, EP was an atheist, I can see the rift between her and her in-laws who were staunchly religious but perhaps wanted more time with grandchildren. This is the situation for a family counselor to resolve. Plus, if EP was extremely introverted, and SP, outgoing, I would not be surprised if he himself had more support in the community. It is not an easy situation for EP.
Regarding SP's mysterious illness. If C.diff or any severe GI infection (salmonella, shigella, campylobacter), home-treated with antidiarrheal drug, ended with pseudomembranous colitis, I can imagine what happened next. (C. Diff is on the rise, btw, and so are all major GI infections. Blame emergence of antibiotic resistance). JMO - EP did not need to poison SP, and maybe, it is his fantasy. If EP was the carrier of C.diff or other pathogen and SP had poor immunity to it, then small wonder that each time he spent time with EP, he felt unwell. All of it has normal biological explanation.
Now, the Patterson's and the Wilkinson's lunch and subsequent death of the three out of four guests changes everything. Nothing, neither dysfunctional family life, nor the need to divide the property, nor potentially the kids' parenting plan could explain the need for three people to die. The dehydrator or not, it is hard for me to explain the input and the outcome without suspecting EP's involvement somewhere along the way.
In short: on the human side, I'd probably tend to support EP till these deaths in her house. Now, she looks like a poi. She has to provide a more cohesive explanation and stop blaming the supermarket. On the other hand, SP's illness could have a more reasonable explanation than poisoning. Also: painters who come to clients' houses to paint walls, take photos and sell them to newspapers are unprofessional.