I don't mind doing it again. Each time I do it, I'm getting better. And I have one browser tab now permanently open to the Navmesh Cheat Sheet, and one tab open to 'Navmeshing the Nordic Stairs.'
Which is necessary, as there were actually 4 yellow border lines I needed to remove, not just one.
http://tesalliance.org/forums/index.php?/gallery/image/28988-navtop2/
There's still quite a bit I need to grasp, such as understanding all the error messages about upside-down-normals or "Triangle 5 is connected to Triangle 6 but Triangle 6 is not connected to Triangle 5." For now I'm just letting the Creation Kit delete the offending triangles, but my goal is to eventually be able to eyeball this stuff.