You’re making complete sense.
With the illness, for some people who have it, time seems to kind of change.
I can ask my family member a question and it might take minutes or sometimes days for a response.
In her mind, she really did plan to help you. You understand that food spoils and you might as well cook it before it does. Just two dramatically different interpretations of the same event.
I guess my advice would be to set a calendar date for shopping and one for cooking. When everyone is getting along and content, explain that the food needs to be cooked in a certain time frame and write those days on the calendar. Then, if she doesn’t feel up to cooking with you, let her know it’s cooking day and invite her to keep you company in the kitchen while you cook. She might join in on cooking. She might sit quietly at the table. She might stay in her room.
I don’t think there could be anything more paradoxical than teaching an adult to be independent.
I think the communication difficulties you’ve come up against are just part of the illness. Calendars and written schedules seem to mitigate for us.