Interesting thread John, good work. Your simplified analysis is sound enough to draw conclusions and it is no surprise you are iterating towards the popular L shape !

Watch out if you reverse the L shape then you may gain for the current horizontal load case, but the spindle mass and plunging forces will cause more deflection vertically so I would say run vertical loading on it, and repeat on the initial design.

In general, as per Eddy's comments, putting the rails on the top and bottom, and moving the Z axis close to the gantry might get a little bit more stiffness, so that would make another good one to try out. Plus lots of builders put an additional strip of steel inside the box section where the rail mounts. This is to get good thread engagement but will also add some local stiffness and help spread the load into the gantry, again adding a bit more stiffness.