I don't know what case law (decisions) there have been on it (maybe Monic will know), but you might want to argue that if you can't remember whether there's food in the microwave, you could forget to check whether it's burnt. Devices that cook are only safe if monitored for things going awry.
It's no proof, but I accidentally killed a microwave forgetting about it and when I came back the food was burnt to a cinder. Would it be worth a search online for news reports about forgotten microwaves and accidents (mine wouldn't be there)?
Also, if you can't remember when you put food in, can you be sure you'll eat it whilst it's still ok to eat? Food that is warmed up and left for hours can be dodgy to eat.
Mind you, what bugged me is that although I got points in relation to food (with exactly the same problems you've got about forgotten food, and a very nastily burnt hand (full-thickness burns), it was very obvious from the report that they didn't read what I wrote, they just copied and pasted from a textbook that said manic depressives can't cook for themselves. So if you can find a suitable entry in their guide for assessors, it's worth a look. I did have a downloaded copy but it's out-of-date (my DLA -> PIP was 2017-18, ten months' saga).