Okay, I believe I had part of the functionality I was looking for all along. Its just that my inventory bag GUI was blocking the player, so I could not see what was happening when I drag/clicked an item outside the inventory bag.

It looks like when I drag/click an active weapon outside the inventory bag, the weapon does automatically disappear from the player's hand, since that weapon item icon is no longer in the active weapon slot of the inventory bag. So, I guess I had that functionality all along without realizing it.

My main goal at this point is to make it so that the item icon (from the inventory bag) that I drag/click outside the inventory bag needs to disappear. This should automatically remove it from the player's inventory list.

My goal after that is to make it so that if the item icon disappears after drag/clicking it outside the inventory bag, that same dropped item needs to appear on the ground next to the player.