The door fits!

After I finished all the walls and paint, I discovered the existing wall next to the stairs was completely out of level. This really complicated things when I went to fit the new knotty alder door and discovered it wasn't going to open properly at all. The existing wall was about an inch out of level from top to bottom, and the other side of the door opening I had redone, and of course, made level. So began the challenge of how do I fit a door to an opening that's completely out of level? What I decided to do was to keep the casing completely level, but cut away some of the wood from one side, and add some additional wood to the other. Thus began the door casing surgery...

I added an additional strip of knotty alder to one side, using biscuits and glue.

After sanding and before any cutting. I set the casing level and
marked the backside to follow the contours of the wall.

Cutting the sides to match the un-level wall.

Nailed into place.

Close look at how far off the wall is, and how much
additional wood had to be added.

Is closes perfectly and is level!

Another look at the wonky side.

A look at the door from the stairwell side.