Still trying to get my head around this one, could do with a few more drawings.

Have you got X and Y axis labelled correct? you wouldn't normally have the Y longer than the X as it places unnecessary force on the Y beam and X bearings.

which leads to the problem of the single bearing on the x Axis, I'm assuming supported rail as that is what you have in the y Axis and that is the a big no no. They cant resist the moment load from the Y axis and also the large distance across the X axis will cause racking problems. you definitely need two bearings on the X axis as wide as the Y carriage.

There is no need the make the C channels in to I beams as it is not needed. the I beam would only increase bending strength which is lower than most designs as you have reduced it by having a beam either side of the load. You pretty much have a resultant I beam from the two C channels.

Personally I would go for profile rails instead of 4 supported rails and 2 ballscrews, the cost would be nearly the same but the strength is much better and can simplify the design so saving money on the other bits.

How are going to do the z axis?