Hi David,
That then has broken Steve’s model because non-manifold edges are part of the nature of the object he is modelling and cannot be avoided. It is simply that this is a real-world situation that Rhino cannot model as a single piece without them. The design element is akin to two cubes placed so they touch at a single intersection line coinciding with an edge of each piece - as in a checkerboard pattern. I’m not at my computer, but it would be interesting to dive into the fine detail and see what Rhino has done with them.
I don’t know how Steve got from your exploded and joined model to the misbehaving surface. In my case, I could replicate the problem by merging all coplanar faces, after which the surface became irreversibly problematic.
Regards
Jeremy