error: you cannot delete an item with 3 edges

[mudplugga]

I did it! I thought of the solution last night in the small hours. In cross section view I rearranged the raked wall flanking the stairs on the garden side around the doorframe using the break line tool and moving the wall’s vector handles. Then in floor plan view I selected the doorframe and deleted it. In floor plan view the wall has jumped back to being a solid wall but is showing fine as a gap in the wall in vector and render view. I thought I needed a doorframe there to make the walkway through the raked garden wall because that whole stairway is in a 'room' without a roof with the stairway resting on the 'floor' at the bottom and linked to a slab at the top. If there's another way to approach this I don't know it.
But I still can’t delete the last vestige of the old polyline in the knee wall though. The jpgs show the gable end as it was and what it is now from the opposite elevation. The jpg in my post of a few days ago shows what the gable as it is now from the other elevation after editing.
Its really helpful talking about this because its only when you describe something in words that an unformed idea changes to a material fact together with your help.