I think it would also help if you get rid of the heat in the cabinet. For sure that heat will warm your sump and then to the DT

For the question, do the rocks retain heat and add it back to the water after the lights go out? No. Once the lights go out, temp should drop again.
For pumps, my MAG 12 is the only thing I use to heat my 20g makeup water tank, it keeps the water at 81. Pretty much any pump will transfer heat, some more than others, submersibles transferring the most.
Fans pointed across the surface will help a lot, but will evaporate a lot of water. This is called evaporative cooling and just for that reason :)
The most stable method to keep temp set is a chiller. That being said, if you only swing a few degrees each day, it's probably not a concern.