One does not become 100lb overweight because of "margins". One becomes 100lb overweight because they are doing 100mph in a 35mph zone so "slow the hell down" is a great advice.
Also important is eating your food in a manner (e.g., smaller portions more frequently) that you don't ever get really hungry. Extreme hunger leads to binge eating which can easily undo any calory deficit you managed that day.
You do not. 100lbs is excess of 350000 calories "stored" and for every extra lb one's BMR increases. By the time someone is 100lbs overweight just to maintain that weight they would need to consume another gallon of ice cream every day.
You are playing with margins when you are 1-5lbs overweight.