She totally could have made it look like an accident!
She should have shown a public interest in similar to mushroom foraging - I.e. foraging for other things, or living off the land - possibly made a social media page about it and made sure people thought it was a keen interest for a significant period of time beforehand. At least 1-2 years. Posting meals she's made with foraged or self grown food etc,photos of her on the foraging trips. She can help by becoming an active member of communities related to this to made it seem extra realistic, if only active on social media groups. Really build the image.
She should have invited the intended victims for at least 1 other meal - possibly 2 - within a period of a year before the deadly meal, where they were served normal food and left healthy. Bonus points if she grew/foraged part of the ingredients for the meals she cooks these guests AND other guests who she invites around during this time. (No mushrooms yet!)
She should have started mushroom foraging very recently - as recently as 3 weeks/a month ago and publicly documented that. Recently enough that it was totally believable she could have made a mistake, but it's totally something she'd be in to because of her well-known and well documented other hobbies. Document this start of this new foraging journey on social media.
If she's going to use bulimia in her defence then she should visit the doctors about this, discuss with friends and family her struggle, possibly even go down the social media route of posting about it occasionally (don't over egg the pudding but maybe just a post to raise awareness during eating disorders awareness week). Ensure she mentions to doctors and concerned friends her laxative use after meals which is a part of her bulimia.
All food served on the same plates but with a very subtle difference like a small chip in hers.
Cook enough death cap wellingtons for everyone - herself included. Cook one with no death caps in. Serve poisoned wellingtons to family members and Serve herself the one without poison but leave one more WITH death caps in the kitchen. Before the meal, cut about 3/4 of this final poisoned one away so it looks like it's been eaten and flush it down the toilet so it can't be found. Leave 1/4 of this final poisoned Wellington in the kitchen.
Eat 3/4 of your unpoisoned Wellington so there is 1/4 left Make a point of showing this to guests "That was alright, wasn't it? But I'm so full I can't finish it" so any survivors could vouch that she had about 1/4 left if asked.
After the meal, take plates to kitchen. Leave them uncleared on the side. Make sure you subtly get rid of the unpoisoned Wellington on your plate (down the toilet!) and replace it with the 1/4 death cap wellington in the kitchen, so if the food remains are tested, it looks like you all ate the same food.
Consume large amount of laxatives so you start to show the same symptoms as the others. Don't clear the plates away! You're too sick, it's suspicious. Don't clean up, also suspicious. Stay at the hospital for a while being visibly very unwell - make sure there are witnessed incidents of vomiting or diarrhea. Make excuse to leave at some point (before blood tests are mentioned) eg need to get back for the kids or they'll be alone.
After the hospital, go straight to bed. Possibly call respectable friend to report how sick you are and ask her to bring something for you or the kids as you're just too sick to do anything. Let her come and witness how very unwell you are. Don't leave the house or bed or clean up for at least 3 days. Have friends come in to check on you / help / witness the sickness during this time.
Don't dispose of anything. It was all an innocent mistake. Admit mushrooms were foraged. You ate them too but went to take your laxatives like you do after every meal due to well known bulimia struggle so mistake have survived. Very sorry.