Well. Three guests out of four dying after a family dinner still strongly points at poisoning.
I am wondering if the poisoner was much smarter than we think, though, and used a totally different product - and maybe, a different vehicle.
Imagine EP thinking far ahead. Simon, stating that she might have poisoned his parents, was looking at a possible explanation.
But what if the mushrooms were not poisonous?
Then, Erin's response was the brushstroke of a genius. She grabs the innocent dehydrator, dumps it into the landfill, et voilà! - everyone's attention is drawn to beef Wellington.
IRL, any poison (what's used in Australia for local rodents?) can be easily added to alcohol. This is something even good pastor would help himself to, but this is precisely kids won't drink, even if they stay at home. And - Erin might be a non-drinking person, too. What do people drink with Beef Wellington? Red wine or whiskey?
So if the vehicle for poison was another food/drink (we have to go through whole menu, including ice cream), a bottle might be thrown away, or even still in the house. So much attention was paid to mushrooms, what was scratched off, who ate what, where did she buy them, that the real poison might be totally obscured.