Hi Phil,

In reading over your long post that’s including a list of many areas of possible issues the high idle speed is a clue to a possible lean running engine at idle. Most likely an air leakage behind the AMM.
The throttle body is not adjusted correctly, shutting down enough or the signal to idle the engine is not getting to the ECU at the appropriate time.
All idle air is supposed to go only through the Idle Control Valve.
Not ahead of the throttle body or anywhere else. It should be able to idle height if it cannot get the air to do so.
Do you have an EGR valve, as it might be stuck?
A open or cracked vacuum line down to the evaporative fuel canister? Sometimes the valve on the can down there can get dirty and stick open.
High vacuum opens at low engine speeds opens both but the EGR is solenoid controlled by the ECU knowing the throttle position at higher speeds.
Things is this are are supposed to transition with throttle position and RPM in programs from startups, warmups and operating temperatures.
This leads to a coolant thermostat error or a temperature sensor telling where the engine is during operation.

Your post covers a wide stance of what-if’s.
Yes the spark plug readings of efficient firing and compression are the basics of setting up the to meet programs of fuel management.
The above is one side of the engine and the O2 sensor is the resulting product on the outlet side.. Otherwise, the fuel system will always try to “retune” very quickly but it has its limitations, designed around a properly maintained set of subsystems covered.

You say the car is acting reliably put the power band is slightly off when compared to other vehicles.
A compression check can tell you only so much about the health of the engine mechanically.
If its appreciably low and evenly then valve timing could be a culprit.
Get the crankshaft up on number one and look to see the cam lobes pointing even upwards might be a get by but you need to know better. You should be able to remove two upper bolts and remove a top screw in the timing cover and bend it back to find a way to see a tiny dot on the front of the sprocket lip to see it aligned to an aluminum notch along the valve cover. Basically at the apex of the curve in the center of the engine. Of course the distributor rotor button should also be under the number one plug wire.
With all those points of reference you can double check things fairly easily IMO.

I don’t know anything about injector rebuilding as they look to me as machine sealed units from the outside. Just pull them and lay a paper towel under them and jumper the pumps on. Replace as needed.
You might have a shrunk O ring on one as a possibility.
The intake manifold gaskets can definitely go bad. That can effect idle speed or power depending on how they fail.

So, how is my list counteracting yours? (:-)
We both sure can post! (:) Comes with the first name I guess?

Phil