Having read the previous thread linked to, I wonder if a child lock would actually work. Could it be defeated by switching the microwave on and off again? I wonder if it might give you a false feeling of security.
Does she have access to a cooker as well?
I'm just asking because when my mother started setting fire to things (in her case, the grill pan) it was a signal that she could not live independently any longer. I arranged (self funded) carers to come in and provide her meals, and they ensured she could not turn on the cooker.