What would make my CL rear rotor move back and forth a few mm a when locked
A fundamental design compromise accepted by Shimano as the price for making something new and proprietary which would still be serviceable by ill-trained monkeys. You can't have no backlash and a sliding fit assembly. As you say, the frequent load reversal which would loosen the lock ring and destroy the splines pretty quickly is not really a feature of bicycle dick breaks. CL rotor/hub interface kinda works for long enough that most people change their bikes before finding out why it's basically inferior to 6-bolt, so Shimano profits keep rolling in.
A fundamental design compromise accepted by Shimano as the price for making something new and proprietary which would still be serviceable by ill-trained monkeys. You can't have no backlash and a sliding fit assembly. As you say, the frequent load reversal which would loosen the lock ring and destroy the splines pretty quickly is not really a feature of bicycle dick breaks. CL rotor/hub interface kinda works for long enough that most people change their bikes before finding out why it's basically inferior to 6-bolt, so Shimano profits keep rolling in.