I would continue with the same crown molding as you have in the room. You end it at the stairs as if you were going around a corner, but it will now instead be end grain against the wallpaper, so there will be a small piece of molding at the end to hide the end grain.
I find it difficult to see from the pictures how the other molding looks, but isn't it possible to use regular door casing? In the worst case, carve a little into the plaster at the transition to the beam.
I find it difficult to see from the pictures how the other molding looks, but isn't it possible to use regular door casing? In the worst case, carve a little into the plaster at the transition to the beam.