Pulse the cranberries in a food processor until fine and no large pieces remain. Transfer to a large mixing bowl and stir in the sugar. Cover and refrigerate for at least 2 hours or up to overnight.
Drain any accumulated juices from the cranberries and reserve for another use, and then stir in the marshmallows and drained pineapple and set aside.
Whip the heavy cream in a large mixing bowl until it begins to thicken, about 1 minute, and then beat in the powdered sugar and vanilla until soft peaks form, 3 to 5 minutes. Fold the whipped cream into the cranberry mixture, one third at a time, until fully incorporated. Transfer to an 8-by-8-inch baking dish.
Refrigerate or freeze for about 2 hours. Right before serving, garnish with the pomegranate seeds.
Use any leftover fresh cranberry juice for other recipes, such as Cranberry Apple Roasted Turkey Breast.