Have only read OP’s comments rather than the whole thread.
Based on the fact that your DD has gained weight rapidly, and you mention a lot of snacking on sweets and other highly processed foods, she is probably dealing with more than emotional eating, though that as well.
She may have binge eating disorder (which is a condition with a defined set of diagnostic criteria), or an addiction to certain types of processed food (which is not generally accepted by the medical establishment, but there is a significant number of professionals who do believe in it, treat it, and are working to have it formally recognised). ADHD makes her more likely to have either or both of these conditions.
If either of those is the case, then a focus on weight, while completely understandable, is not helpful. Either from you, or her. The focus needs to be on understanding and dealing directly with the dysfunctional eating patterns and what lies beneath them. If she is able to do that then the weight gain will stop, and she will probably lose a fair bit of what she’s gained.
I don’t know how you go about it, OP, but I can share some of my own experience. I was a fat child, obese teenager, became morbidly obese in my mid twenties, peaking at over 22 stone in my mid-forties. This relentless rise was punctuated by frequent diets, none of which got me all the way to goal, but all of which were followed by a period of rapid gain.
For the first 30 years I just thought I had a weight problem, but in my early 30’s realised I had an eating problem. I met the criteria for Binge Eating Disorder, but never found an effective treatment.
My solution now is Overeaters Anonymous, which I’ve been part of for coming up to 11 months. The few months prior to joining, I was in the familiar rapid gain phase following a major weight loss. Joining OA has put a stop to that, and allowed me to very, very gradually lose most of that regain. Although I remain overweight, the yo-yo has stopped. I have been wearing the same (smaller) size of clothes for 18 months now, which has never happened in the previous 60 years. I eat sanely and normally and have opted to completely eliminate sugar and processed snack foods because of the way they trigger my compulsive eating.
So my suggestion to you, OP, is that you google phrases like binge eating disorder, compulsive eating and food addiction. Have a read, and see where that takes you. If I’m right then there is every chance your DD is eating far more than what you see and hiding it because she is ashamed. She thinks what she’s doing is disgusting, and assumes you will too. Listen to the stories of people who suffer from these conditions, and who are recovering from them. You may initially be shocked by some of the things people do….. but it will help desensitise you so that you are not shocked if and when she starts to tell you what she has been doing. She needs hope, acceptance and love.
Recovery is possible.