I presume you already know that the engine computer can’t figure out why the amount of gas required to inject to keep the pre-cat O2 sensor(s) satisfied doesn’t match up with what it would expect based on these other sensor readings and factors
- throttle position sensor
- Intake air temperature
- Engine Coolant temperature
- Altitude/barometric pressure
- the fuel injector flow is within spec,
- the fuel pressure at the fuel rail is correct
- and properly adjusted properly up/down for intake manifold vacuum
- the ignition system is working within spec, good spark and proper ignition timing
- the valves are opening and closing on time and opening to the spec’d amount
- the cylinder compression on all cylinders is within spec
- the engine computer is working correctly
- the engine is air-tight from the air filter housing to the tailpipe
So you just have to go through each of those, one by one. Eventually you’ll pretty much have to figure out what’s causing the problem. You appear to be off to a good start already. However, don’t entirely discount the possibility that something you’ve done has inadvertently made matters worse. That’s something easy to do since it isn’t always possible to test each factor independently.
Based on the unusual results of the fuel pump experiments you’ve done, my guess would be something is amiss with the fuel system. the engine computer assume if it pulses the injector a certain number of msec, a certain amount of gasoline will make its way into the cylinders. If that amount doesn’t actually make it, you’ll get a fuel trim error. Start with the fuel pressure. Since this problem occurs at idle, that makes things simpler, all the testing can all be done in the driveway, no need to try to measure fuel pressure while you drive the car. check the fuel pressure at different manifold vacuums levels too, it should change directly with the manifold vacuum, the higher the vacuum the lower the fuel pressure and visa-versa.
If no problem with fuel pressure, next up would probably be a fuel injector balance test. The rail is pressurized to spec, the pump shut off, then the injector under test is pulsed a certain amount of time. The fuel pressure should drop a certain amount. You’d need to consult the factory service manual for the details of how to do that test. But it would tell you whether or not each injector was injecting the proper amount of gasoline under a controlled test.