I would like to see options added to the context menu in Tracks, chapters and tags: on the Multiplexer tool to mark or unmark all selected items. I see the option to select or unselect all, but that does everything in the list.
Welcome!
If by “mark” you mean that you want to toggle whether or not an item is copied: either press return or toggle the “Copy this item” combo box in the properties pane. All changes in the property pane apply to all currently selected items.