Using a Motor Controller to Limit Voltage to a Motor
Yes. A PWM motor controller is a buck converter: an additional buck converter would be redundant. With a reasonably efficient motor controller, the system is likely to be more efficient since the losses from the buck converter have been eliminated.

Just don't burn up the motor by driving too much current through it, for too long. The resistive losses in the windings generate heat which will eventually burn off the wire insulation or cause mechanical failure. Current in excess of the motor controller's specifications can also overheat the controller. This is true regardless of the battery voltage, but a higher battery voltage makes it easier. Many motor controllers have a current limit feature for this reason.

Brief periods of high current, or even 24V applied directly to a 12V motor  are not problematic as long as the motor doesn't overheat or exceed its mechanical limitations.