If you're using multiple player groups (via group IDs), you can swap members between them using a Transfer menu item in your Group menu part. The Group Combatant menu part can display information about the currently selected combatant in a Group menu part.
The regular Combatant menu part is limited to the current active player group (same as HUDs), so to display information for a separate player group, you'd need to switch the active player group - e.g. in the Open Game Event of the menu screen (and switch back in the Close Game Event).
There's no confirmation dialogue for removing items from the inventory menu, so I guess that's the quantity selection. E.g. removing via the sub-menu has the setup for how quantities are selected in that sub-menu button.