The IAC is controlled directly by the EEC, so you can't tune that without an EEC tuner or chip.
The EEC does adjust for whatever the base TPS reading is when you start the car (so I have been told). So the TPS has some self-correcting capability, but it should read in a certain range.
I think the TPS output voltage should be from 0.7 to 1.0 volts with the throttle closed, and 4.5 to 5.0 volts at WOT. If you have an analog voltmeter, you should also watch the needle sweep as you slowly open and close the throttle. It should move smoothly with no dips or jumps. If the reading is off, you may be able to slot one of the screw holes in the TPS sensor itself and move it slightly to compensate - but you shouldn't have to do that under normal circumstances.
It's a bit of a long shot, but you may want to check that the minimum idle is set correctly. Unplug the IAC and try to start the car. It should be able to hold an idle around 600 rpm. If it dies, turn the car off. Got back under the hood and turn the minimum idle screw so as to open the throttle a little tiny bit. Try again until you get there. (Don't adjust the screw with the engine running or IAC plugged in, because it will try to compensate.)
It really sounds like a bad IAC, but I know you changed it already. Maybe your wiring is loose or corroded? Or maybe the EEC is getting a bad signal from the cam or crank sensor.