I like your U-shape idea to keep the depth of the build down to compensate for the tool changers. When 1st looking at your Z, I thought it had 1 lead screw. After looking at all the pictures closer, looks like that is stationary & only the top part moves X,Y & Z. That looks pretty cool.
I think you were right the first time, the top plate with the xy motion is static, only the bed moves in z. The size and quality of the Z linear rails are such that one lead screw should be sufficient for the cantilevered design. ive uploaded a couple of photos of the hardware so you can see what i mean.