I had a look at your ball shifting shaft and the shaft itself looks fine. I found it interesting that you made the shaft wall 1/8 thick so the balls can't fully engage, that might be a problem with wear depending on gear material and tolerances but it could very well be totally fine.
What I am very concerned about is bearing load. Its seems you're using R1212 bearings and according to lily bearings (
http://www.lily-bearing.com/ballbear...12-bearing.htm) they only have a dynamic load rating of 111 newtons and static rating even lower. These numbers may be very conservative (or just wrong) but I would recommend doing some math and researching the ratings of the actual bearings you'd use to see if they'd work. If not, look into bushings or bigger/different bearings.