This is a complete thought experiment, but: what if they didn't?
Technically Don's might not have, as we know he ate half of his wife's as well as his own.
I have noticed in testimony that Erin seemed to have a particularly close relationship with him and she has spoken repeatedly both on the stand and in the early interview by the car about how much her children loved him.
It could have been he was intended to be spared but things went awry when he decided to eat Gail's leftovers.
In that instance, could it explain Heather & Ian being invited - so that not ONLY Gail got sick, making things look less targeted on Gail?
It's a bit "out there" but technically plausible.
Technically a plausible theory, yes. But people only survive if they get rapid treatment. If she had wanted one to survive, she needed to go down the path of quickly "remembering" and admitting to using foraged mushrooms so that they could be treated urgently.