I don’t find it hard to believe EP wanted her ex dead and at a stretch I can see she might have wanted to kill her in-laws. But Ian and Heather? Why?
And my impression is that she was not expecting the police to be onto the mushrooms. Certainly not initially and perhaps not at any point. But how was she expecting to kill 5 people from food poisoning (her ex was originally supposed to be there) and survive herself without a huge investigation that would lead to the outcome it has?
Does this lend to the theory that she is innocent? Or was she not expecting to kill the victims? It’s difficult to make sense of.