I think it was me who said this, and the reason IMO could be that if the conversation about EP possibly reconciling with Simon was met with resistance from the dinner guests then this could have been the deciding factor to serve the poisoned dish IMO. If the guests said they’d be overjoyed at a reconciliation and would try to encourage Simon to get back together with EP then that could have been a deciding factor to serve the poisonless dish.
Personally, I’m wondering whether EP was the one who wanted a reconciliation and Simon perhaps wasn’t having it. Maybe EP invited his parents and aunt & uncle round as a last ditch effort in the hope they’d be willing to try and talk Simon into reuniting with her. Maybe her request was met with resistance and this angered her, the knowledge that her final attempt had failed could have been the ‘straw that broke the camels back’ and the decision was made to serve the poisoned dish to her guests. Leaving the unpoisoned one(s) for her children to have as ‘leftovers’, with the button mushrooms she has mentioned scraped off.
This scenario would explain the 2 different types of mushroom, one apparently (according to EP) purchased at the Asian food store and the other from a supermarket.
I think there is usually a little truth in every lie. Perhaps there were 2 different kinds of mushroom, that were used to prepare two ‘versions’ of the beef Wellington. Perhaps the children did consume beef Wellington with the mushrooms scraped off - but maybe it was the alternative one she had prepared and would have served had the conversation went her way. This is all JMO
ETA - Perhaps when EP learned that Simon wouldn’t attend the dinner, she became incensed and decided if she couldn’t get his family on board, then she would punish him instead. Had Simon attended, I don’t think there would have been any poison in the meal, personally I’m wondering if it all wasn’t a case of ‘a woman scorned’
MOO