"If I already have everything that's in the grab bag, what happens? Do I get tradeable versions or something?"
Got my answer. Thanks.
If you have acquired every collectible but still have Grab Bags to open, they’ll contain Group Repair Kits instead—useful! For this event, a Grab Bag can contain Hollowjack Motif pages, the Skeletal Marionette memento, Apple-bobbing Cauldron fragments, and the Spectre Mask collectibles.