RE: Does the rear heater add to engine cooling?
I'll answer this from direct experience.  I had an '85 with cooling
problems (later replaced the radiator).  On summer trips on the
highway, climbing mountain passes, I had the rear heater open (and the
front heater also) with the fan on max for the extra cooling.  When I
turned the rear fan and or the front heater fan on to full blast you
could see the temp level drop.  Kind of like Feynman and the ice
cubes, run the experiment and get the answer.
BTW, this is also a good way to melt the fan switch on the dash (did
that once) and if you don't think the rear heater can pull some heat
out of the system you should have seen the melted plastic grill on my
rear heater.
 One more note.  I have the rear heater pulled on my '90 but have
never needed the additional cooling capability.

-- 
Loren Busch