Overeating can be related to mood issues, specifically anxiety, depression and bipolar disorder. The only way it is related to ADHD is if the child is not hungry during the day from taking stimulants (which decrease appetite). When the stimulant wears off in the afternoon or evening, the child is very hungry and eats to make up for not eating very much all day.
If I remember correctly, your difficult child has a fair amount of anxiety. I'm guessing she's eating to self-soothe. Or if she has Obsessive Compulsive Disorder (OCD) tendencies, she could be eating compulsively. In addition, boredom in young children is often a symptom of depression. I'd recommend asking your pediatrician for a referral to a child psychiatrist.
