Just a couple of thoughts and observations, for what they are worth..
The voltages you listed are goofy. A fully charged "good" battery should measure 13.2 volts (at least a lead acid battery..). In a car, hooked up to Some load (dome lights, etc) it'll be a bit lower, but measuring 12.1 or 12.2 is not good. And why/how would it measure lower at the Alternator terminal than at the battery?
Then, after that, it's overcharging. Showing 15+ volts is not right, not unless the battery is screwed up or there is a major ground/connection problem someplace. Once it settles down, warm and "normal", it ought to measure around 13.5 or so, at the battery. More than that indicates a problem with the battery or the charging system, somewhere.
The EST and the ignition module do not have a lot of over-voltage compensation built into them and That could be your real problem here. It (high voltage) could also be affecting the ignition coil..
Beyond That issue, a sticky or otherwise messed up EGR valve is a very likely cause of idle speed problems. Disconnect the darn thing and Plug the port (at the plenum) - eliminate it from the equation After making sure that it's really and truly closed. The EGR control solenoid on these years is also a potential problem - they die.
Verify, if you haven't already, that the min idle set procedure has been done, and that the TPS has been re-adjusted to a .50 - .54 volt reading afterwards. It matters, a Lot, that the throttle body min idle stop adjustment is at least somewhere Close to right, and it matters even more that the TPS is showing 0.50-.54 (and I know that the Book says .54 +/- .07, and it'll Run, but higher than .54 is not going to help the idle stay steady..) to the ECM when the throttle is closed. If you want it to start right, and if you want the idle to be nice and steady.
How sloppy are the bushings in the throttle body, or to say it another way - how sloppy is the throttle Shaft? Up to a point it won't make much difference to how steady the idle is, but beyond that point it will make the idle very unstable if the throttle shaft is really slopping around in worn out bushings.
BTW - what do you mean by "replaced the voltage regulator"? There is no voltage regulator other than the electronics built into the alternator... Well, there is a step down as part of the ECM main board, to drop the 12 volt (nominal) supplied to a 5 volt reference level. And tapping or jarring the ECM is Never going to be a good idea, really.
Oh - but tapping on the Block (or heads, or whatever) to verify that the knock sensor is working right might be a good thing. With a timing light on the motor, and the timing connector Connected (so that the ESC system is fully functional), tap (doesn't take much, or shouldn't) on the passenger side exhaust manifold with a smal hammer - the timing should jump when the ECM "sees" the sound.
That's another thing that Might be causing your idle issue, although sort of uncommon. Try simply disconnecting the knock sensor, down on the passenger side of the block. It'll eventually set a code, but not right away, and meanwhile if it was falsely telling the ECM to retard the timing that could be the deal..
As for telling if the harmonic damper has slipped and if the timing marks are off as a result, well.. The only way to do it is to find TDC, #1 cylinder, Mechanically. Accurately. Then see if the timing marks are where they should be. Considering that the damper (if it's original) is 25 or 26 years old now - my money is on the Slipped side. It would not be at all unusual. Set the timing "by ear", and forget about the light and marks. Ignition advance is Not going to be the root cause, nor is it the solution, to an unstable idle.
Almost forgot (OK, Did forget..). Unhook the oil pressure sender (the One wire sender, the Two wire thing is the fuel pump switch) and see if the dash display changes. Then jump that wire (still unhooked from the sender) to Ground and see what the dash does. Assuming that the Dash display goes thru it's range then the sender is bad. If the dash display does Not go from one extreme to the other then there is a problem Other than the sender, like in the wiring or in the dash electronics. The sender is the most common failure, at least. And that circuit can't effect anything else, either. And yes, I know that getting down in there to that sender is a PIA. It's easier with the distributor pulled out of the motor