DC-DC converter to drive DC motor
Our intention is to use a 24-12V DC converter and add a battery in parallel to handle the reverse current surges.


You can do a lot better. A motor is a DC converter already, so to run a motor from a DC converter is probably more expensive and less efficient. All you need to do is find a motor driver that can run on 24V, then configure it to have a current limit such that your motor doesn't overheat. It's excessive power and the consequent heat, not excessive voltage, that is the usual limit for motors. If you get enough voltage to cause arcing in the windings that can be a problem too, but 24V isn't anywhere near that limit.

I'd also consider more robust overvoltage protection. A battery's ability to absorb energy from the motor is limited by its internal resistance, the inductance of its leads, and its current level of discharge.

Do some analysis of the mechanical energy your system can have at a maximum, and then work backwards to see if your battery could absorb that in a very short time without driving the supply voltage high enough to break stuff. Also consider what happens if your vehicle is driving down a hill: in this case your battery must absorb the gravitational potential of your vehicle faster than friction can slow it down.

Then, add something like a crowbar circuit to handle pathological cases or to save yourself the cost of a new motor controller if your calculations were off.