What is the most efficient way to prevent reverse polarity to an opAmp?
A series diode on the +24 would help. Putting a diode in the ground can cause more problems with external signals. Right now your comparator switches at about 11.75/12.27V. With the diode it would switch at more like 11.45/11.97V, so not much different. 

You might also want to protect any other inputs and outputs against connections in any possible combination.