I haven't posted in this thread before, so delete if inappropriate. I have a theory that I've posted elsewhere, but a lot more discussion is generated here.
So much of what Erin has done and said, post the poisonings, has just not made sense. For a smart woman, there had to be a smart plan. We know it was thrown off track, but how badly? I find it hard to believe she planned on having her life torn apart, even if it was ultimately determined to be an accident. One explanation could be:
We know she planned the lunch for 6 people, Simon included.
They were all to experience the initial symptoms, assume gastro, stay home or seek minimal medical intervention. Then they would have deteriorated and either died or become so sick at home, that no-one would have been capable of providing any details to authorities. Any conversation would have been amongst themselves, so color of plates, cancer story, even individual beef wellies etc. would have remained unknown to anyone else.
With everyone dead or desperately ill, she can then tell whatever story she likes, within the bounds of any remaining evidence. I don't think she ever intended to be even blamed for accidentally poisoning them. We know that Gail (or Heather?) brought an orange cake, obviously in a container.
I think her version was intended to be along the lines of: I made the beef wellington, vegies and dessert, but Gail (or Heather) brought along this lovely mushroom gravy. The others all had that, but I had packet gravy as I don't like mushrooms (or similiar). We know now that the DC's were inside the pastry, but we wouldn't if they were all dead.
The orange cake would be gone and the container can be said to have contained the gravy. She had likely planned to add some of the contaminated leftovers to the container to make the evidence fit. She not only would have killed them all, but one of them would be blamed for the accidental poisoning. She would have minimal explaining to do to police, before they focused elsewhere. The story would have been about how lucky she and her kids were (eating leftovers), that they didn't eat the toxic gravy that grandma made.
2 things went wrong. Simon not coming, but too late to back out of the plan. Then Simon taking them to hospital whilst they were still capable of talking. She left that hospital knowing she was in all sorts and made up the Simon/dehydrator story to explain her panic. I think that could also explain the apparent trip to Loch after leaving the hospital. Some extra DC residue to add to the container could have been her initial panicked thinking. I think it would also explain why she invited Ian and Heather. She couldn't afford the sick Gail and Don telling them any of the details before they died.
I don't think she had really planned too much at all beyond the lunch, as the finger of blame (even for an accident) was meant to very quickly point elsewhere IMO. She then had to adjust her story on the run, based around what she was confronted with, which is why it is such a mess compared to her meticulous foreplanning.
We will probably never know if there is any validity to the above, barring a full confession one day. It doesn't alter her guilt/innocence, but IMO could explain a number of things that haven't made sense. MOO.