Neat! that Hbot video is impressive - Looks like a clever idea, if there's an easy way to turn a moving motor into a stationary one i'm all for it. I'm not sure i completely understand how it works, but it seems to me if its really only a single long belt, then there's no way the drive belt can support the gantry and hold it rigidly perpendicular to the side rails - i.e.resist against racking like the moving knot cable stay arrangement does?
I can see it working as advertised in the "X" configuration, since it is naturally cantilevered, but with a gantry "H" system i see possible issues with racking unless i'm missing something obvious, which is a distinct possibility
If the Zaxis getting out of square is as much an issue or annoyance as you say, then in my mind that would tip the balance in favor of trading the 2nd motor+related bits for a pair of timing pulleys and belt, or equivalent even if the cost is a wash either way. But if you keep the 2nd lead screw that way you'd have to replace the motor with a bearing.
Of course never having played with any of these things myself yet, this is pure speculation on my part