The guests helping themselves, choosing their own plates and the fancy beef wellington, could be explained by her carving the fancy pastry encased beef at the table and the guests then either helping g themselves to a slice OR her saying who’s next and the guest lifting their plate for her to pop a slice on their actual plate.
I do feel it’s a whole new level of pre-meditated to think she picked the mushrooms fresh, in season, months ago, dried them and kept them.
It’s a possibility, some people are warped. IMO
Alternatively, could it be a possibility that one single rogue death cap had found its way into her food store? Either by her own hands or because she bought dried foraged mushrooms elsewhere. They seem recognisable fresh, would they be so dehydrated?
Lastly, if it was premeditated, the whole thing gives me very ‘Last Supper’ vibes.