Hey Wayne, I got both arms rigged up and the work perfectly ... except for one thing. And I'm at a loss for what could be the problem.
It looks like my toggle for the IK_Hand_parent is working backwards. Set to 0, the IK hands are locked to the movement of the chest. But the elbow lock is locked at 1. I checked out the final blend file and your file has the hands and the elbows locked at 1. I've gone through all the parenting, constraints and the drivers and I can't seem to find what the issue is that is reversing the hand toggle. It all works, just that one custom property is backwards. Any ideas?