OK I read back over the whole thread. I mentioned the procedure for setting the "BASE" idle, but never explained it. Did you find this somewhere? Just so I don't take anything for granted, here's the cliff notes...

First things first:
Find & fix all vacuum leaks. Get rid of any air from the cooling system.
Throttle cable MUST have a little slack.
Mechanical stop for the throttle should just barely prevent throttle from wedging stuck closed; this should not be used to adjust idle.

Warm up the engine completely. ALL electrical loads switched off.
Unplug the wire from the IACV (#9 in the picture).
It'll almost stall; adjust idle screw #1 for 500 rpm.
Turn it off & let it cool down & plug the IACV back in.
Reset ECU (pull fuse #13 at right-side end of dashboard).
Start the engine & let it warm up without touching the gas pedal at all.

If that's no good, maybe your throttle-position sensor is bad? It's not numbered, but it's located on the throttle shaft opposite the cable pulleys. Look for proper voltages, I'm not sure of them. Something like 0.45v when it's closed & 5.0v when it's open & smooth variation in between?

If you have to close the idle valve (#1) all the way, then maybe there's a vacuum leak?

ps... When you turn on the AC, it shouldn't surge. It's supposed to add enough air to maintain about constant RPM. If the system did NOTHING, then AC would cause the RPM to drop or stall.