Could it be that the engine just needs to increase the idle when there are electrical accessories turned on to compensate?
Pretty much but increasing the idle mightn't be the best/easiest way.
An alternators output can kick in at various speeds but most start to put out power at around 1000-1500rpm (not crank rpm).
They also put out more current (amps) the faster they spin up until a point. (Google "bosch alternator performance curves)
At idle (low rpm) your alternator is making very little current.
Reading back to the start of this thread you seem to have alot of electrical accessory load (ewp, ecu, lights, fuel pump, thermo, battery, etc).
This is probably too much of a current draw when at idle for your alternator to supply.
I'd now try one of the following:
-get a bigger alternator that puts out more current at the same idle rpm.
-put a smaller pulley on your current alt to make the guts spin faster at idle therefore producing more current.
-like you said, raise your idle like factory setups do when you turn on your a/c...