The hallway rules are a different rule than rooms. You are only required to have 1 receptacle in the hallway, if the hallway is 10 feet or longer.
Having said that, I think the one in the turning space is a good idea. It would at least give you a convenient receptacle to plug into when vacuuming the stairs.
How are you going to decorate the hallway, if at all. If you might set a table somewhere or hang a plugin light on the wall (or over a picture), you might add more receptacles to accommodate those. But for a plain hallway, the one receptacle will do it.
Having said that, I think the one in the turning space is a good idea. It would at least give you a convenient receptacle to plug into when vacuuming the stairs.
How are you going to decorate the hallway, if at all. If you might set a table somewhere or hang a plugin light on the wall (or over a picture), you might add more receptacles to accommodate those. But for a plain hallway, the one receptacle will do it.