When you started talking about scanline interrupts, I had thought that you were going to start talking about the "Racing The Beam" book at some point.

The only reason I can think of the horizontal or vertical method of the tilemap was due to the simplicity of games like Super Mario Bros, where you could only move to the right. My guess is that they at least gave the ability to move in two directions (left/right or up/down) but maybe never considered that developers would want all four directions (i.e. "640k should be enough for everybody"), so it required some software finagling as you've mentioned above.