so your saying a vent under the car would push air up, and vents in the window will pull air up, therefore your using the rear quarters as exits for air? aswel as the boot, why not use the roof, the roof has a lot more clean fast flowing air compared to anywhere behind the rear of the car!
in through the roof out through the suction of the bottom of the car, both have fast moving air.
whats stopping you pulling the intakes up to under the seats, up the natural upwards directions where the rear seats used to be in the centre of the car and back out the wheel arches?
think about wheel arches, the ph1 clio's used to use "clio turbines" which were wheels shaped like turbines, dno if they worked to move air from under the car to the outside but why cant this idea be used here, using motion of the wheel to suck air out to the outside world.

or on the window front again, modify the rear windows to open INWARDS (it can be done) encase these up into 2 separate rads! and back out the rear A pillar window, or boot sides below the window.
here you would ensure a higher area of intake-exit of the air flow, and it means not having to ruin under-floor aerodynamics.

theres many ways around this, trial and error will show u the right way.
as someone has already said, get a shitter and play with the bodywork till you find something that doesnt overheat, then scrap the thing and make something nice for your own car