I think the handle could be strengthened in this way:
 Currently the outside handle moves the lock mechanism by a #6 socket screw in a threaded hole in the handle shaft. It's inserted until the shank butts up against the shaft- this means that the stress is concentrated at the end of the threaded portion. The screw is not in shear, it's cantilevered as it passes through a slot in the collet and then into the actuating drum. It looks like the break occurred at the end of the threads.
 The screw hole passes through the shaft and is threaded on both sides. The upper hole could be bored out to fit the grip of a #6 screw leaving the lower hole threaded. Unbrako makes a #6 x 2" socket screw that has a 1" grip- allowing about the same amount to protrude as the screw does now. The longer screw would bottom out on the inside of the handle shaft tube and the excess thread would be ground off so it was slightly recessed into the shaft OD. The result would be improved strength with minimal fuss.
 Comments?  -Bill Wade