Georgie Boy,
Your right on the money with your elaborate scenario of coolant
radiation in conjunction with the size of the radiating surface
involved.

I have no problem with the size of the radiator and the amount of
cooling it's going to do.

Picture this;
Remove the 2 core radiator, and add a 4 core.
More tubes, more area for the fluid to be passing though, more air, more
cooling.

Now, instead of adding that 4 core radiator, just add two more smaller
units.
Much like adding Hayden accessory trans cooler's to any automatic
transmission.
( which by the way uses no fan, it's dependant solely on vehicle
movement for cooling )

Your giving the fluid more area to cool down in, more area for coolant
to pass through, and hypothetically more air exposure, and cool down.

It doesn't make any difference where it's located .

So in retrospect, using the front heater core, as well as the rear
heater core ( with the fans on ) will drop the engine operating
temperature.

The rear heater won't do much without the fan on.
Correct you are.

But crank that valve open on the front heater at speed and see what
happens.

Your vehicle has some problems if the temp rises when you just open the
rear valve.

Sell it quick, it's gonna blow up.