Hi

Though here is what i think of it in my case, may be concerning this build also:
i agree with what your saying here, am thinking i may cog and belt the 2 ballscrews together, to stop any small chance of them going out of alignment. Or, go back to using one motor to drive both ballscrews.

I still don't like that design of yours, but that shouldn't discourage you :-)
Dont worry me, i prefare people to be direct and to the point. and getting something wrong is not failing, its a leaning step on the way to success.

I don't know if i am understanding you wrong. But:RP is far more wear prone than ball screw.
I was thinking aloud, i am going with ballscrew,

You can not bend with steppers a properly constructed machine.
.

I have seen this happen, Nema23 on z, Z bocx mounted forward like you pictured. Z down gave it enough leaver, that when his limit switch failed, put a slight bend in a very well constructed Y gantry, both the mounting rails and ballscrew had a noticable movment when passing the point were it had bent.

He was very ham fisted with his limit switches, hence it failed, but it did bend it, i will have a go at doing the math tonight again,

IMHO the weakest points in order of a DIY machine FYI:
1. Router bit - deflection
2. Z overhang-plate twist
3. Gantry twist and bend ,
4. Gantry sides
5. bearings/assuming you use square
6.Spindle power
7. Balscrew support bearings, spindle bearings
8...etc
Wen i consider the list of week points, i think of it more the order i would want it to break if it does go wrong. I need to put my design is sketchup so you can see it,

am thinking ( but will acsept that am wrong, ), is that No 1 should be the baring mount screws or the baring, I think you will understand when you see my X design.