I think there is a strong possibility that Simon was in it with her. I don't think she's innocent but it doesn't make any sense that she would do this alone.
Why would she even organise a lunch with her estranged husband's relatives at her home?
He's the one with more motive as he stands to inherit. When he inherits then he'll no longer want or need a share of Erin's inheritance when they divorce (which also gives her enough motive to go along with it).
He pulled out of the lunch at the last minute in case he accidently got poisoned too. He knew what was on the menu.
He alleges that one of the victims told him Erin's food was on a different coloured plate, conveniently Simon is the only person to have heard this piece of information - if you were seriously ill on your way to hospital, would you even remember what colour your own plate was, never mind anyone else's? The surviving guest can't even remember if the food was served on a tray or on the kitchen bench. I think Simon made this up to further incriminate Erin.
He asked her at the hospital if she used the dehydrator to kill them (though he now says he didn't say that). It seems really obvious that he knew exactly what happened before the doctors did and before any toxicology tests came back.
He was also a mushroom forager, have the police looked into his whereabouts in the weeks and months leading up to this lunch? His internet searches or Facebook groups?
Why would Erin need a burner phone? Who was she calling to make plans/arrangements with? If she was doing this completely off her own back she wouldn't need to call anyone from a secret phone. Perhaps Simon also had a second phone but nobody thought to check.
It's easier for a jury to believe that it was a terrible innocent mistake for her to feed them poison mushrooms as she had no reason to want them dead. She possibly agreed to take the blame since she could feasibly pass it off as an accident therefore receive a lesser sentence. If he had attended the lunch and not gotten sick it would have looked more suspicious.
I know there are some of you who are absolutely convinced that Simon is 100% innocent but I think this shows that there's room for reasonable doubt.