Thank you for your kind words, glad you enjoy it!
I can’t tell you about the double tap because I don’t have mobile development experience but I would wonder if there isn’t a way.
This would be super easy. Not an inventory slot but just create a new widget. Override the function OnDrop. In that function cast the Operation to BP_InvItemDrag, if that is successful call the function UseItemInSlot Of BP_PlayerCharacter with the Inventory and Slot variables of the BP_InvItemDrag operation. You will also need a GetPlayerCharacter node and cast it to BP_PlayerCharacter in order to be able to call the function UseItemInSlot. Now this should already work.