I apologize for not reading all the posts, so someone may have mentioned this but here goes.
I believe it is universally accepted that doorways, and especially stairways are really bad places 'funnels' to have to pass through when clearing a house. If everyone is upstairs and safe, why not put that 'funnel' to your advantage by using it against your invader?
The last place I would want to be is ascending a stairway with an armed opponent at the top. He would have the advantage of some cover and concealment, elevation, and reward mobility. I would be stuck with almost no good mobility or cover.
The top of the stairs seems like a great defensive posture to me. You have to pick a line somewhere in your house. It might as well be the at the most advantageous place in your house.