A half-century after dropping 70 pounds and keeping them off, Weight Watchers founder Jean Nidetch made some allowances: Cokes in her fridge, Klondikes in her freezer, the occasional potato or extra piece of bread on her plate. But she never again touched the chocolate marshmallow cookies she called her ultimate weakness, the treat she’d stash in the hamper and eat by the boxful in the middle of the night, all the while praying she’d choke on her next bite.