Using a food processor, pulse the cranberries and sugar until fine. Transfer to a container and refrigerate for at least an hour (or overnight). Using a sieve, drain the chopped cranberries (my kids love to use the reserved liquid for a fun drink with sparkling water) and combine with the drained pineapple and the marshmallows. In the bowl of a mixer, whip the heavy cream, and powdered sugar to soft peaks. Gently fold into the cranberry mixture, along with about 1/2 cup of the pomegranate seeds. Garnish the top with the remaining pomegranate seeds and refrigerate until ready to serve.