Well to monitor the speed of fans and the temperature of differenct pices of hardware (or water) the Aquaero was invented.
You would need an Aquaero and an aquaero plug&cool-Temperatursensor. You can plug the "Temperatursensor" in every unused Plug&Cool adjust.
Or in a T-connector if you like.
To something you wrote:
I'll switch the fans underneath the radiator so that they draw air out. And I'll fit another 2 120mm fans on top of the radiator, to push air through.
As a matter of fact, there is just a very little difference in using the fans with 12V current and with 5V current.
But it's a slightly higher effect for you ears

AND: You shouldn't mount fans on top and beneath the airplex as long as the fans aren't turning the same way (e.g. all fans on the airplex moving air in the same direction.)
I would recomand to you, that you would mount both fans beneath the airplex and let them blow air out of your case.
(more silent)
AND: Mount one of the Temperatursensors (not the one I've mentiont above but one of the sensors delivered in the Aquaero box) on your GPU if possible -> it will show you a more accurate tempreture.
I appologize for my bad English.