Hi Vic.
Nice work, Looks very well thought out. I like the idea of using ali channel instead of the profile bars. There was a thread looking at an open source mid range Ali router machine and it looks like this could be it.....

not sure on your axis terminology but i take it by y axis you mean the longest axis? if so then you could use belts or wire to control the racking and stil use one ballscrew. If you increase the distance between the bearings then that will also reduce the racking but require longer rails or reduced travel.

My main concern is why use c channel for the rail supports if the ballscrew is in the middle? it would be much stronger to use rectanglar box section, I would at least provide support at the ends and mid point.