If I want to use a custom controller, set the built-in PID to 0, and control the motor in torque mode using Protocol 1.0 for Dynamixel MX motors in C++, can I do that?
You can control the actuator in this way, however even setting all the internal PID controller parameters to 0 will not disable the PID control functionality. This might not produce the motion results that you are looking for.
I would instead recommend upgrading your actuators to 2.0 protocol and using PWM control mode to bypass the PID controller.