Could it have been that she didn’t have those leftovers poisoned in the first place, but then staged that part after the meal?
I mean, could it be that she had poisoned the four portions for her guests from the start, but left bw 5 and 6 without until after the meal? It has been said she was very frugal and we know eye fillet is a more expensive cut. So, maybe didn’t want to waste more of the expensive meat than necessary. She had laced the meat of the 4 guests she was sure were coming with dc poison (maybe a dc solution applied to the individual pieces of meat with a syringe), but didn’t poison her own and also not the 6th bw after Simon pulled out. Then she ate her unpoisoned half portion during the lunch and fed the meat of the unpoisoned 6th bw to her kids the day after and put the rest of the bw in the bin When she came back from hospital after having been confronted that there is a suspected dc mushroom poisoning, she laced the meat in the half portion of her own leftover with a bit of dc poison to make it look like an accident and pretend her portion was just as poisoned as the bw of her guests. As she didn’t expect the poison to seep from the meat into the mushroom paste and the pastry, she thought there wasn’t a need to put any poison in the meat free leftover of bw 6 and left it as it was.
Sorry if my writing isn’t very eloquent, I’m not a native speaker.
IMO