Using 3 pages, the plug-in won't allow us to right-click or hover loot from our inventory to put items into the backpack if the first page is full and the others are empty.  Is this the only way this works or is it possible to allow overflow to automatically be moved to the other pages, even if it's not the page showing?

Each page technically uses a separate container, so you must view a given page in order to right-click or hover loot items into that page. It would be possible for me to update the plugin to hook the player attempting to move the item into the backpack, in order to apply better logic, but the hook I would have to use is frequently called and highly used by plugins, so it would have performance consequences and high potential for plugin conflicts, so I don't see myself doing that.

Note: Gather mode is not subject to this limitation.