Hi there!

Check the fuel pressure regulator that is mounted on one end of the fuel rail.

If the vacuum hose smells of gasoline or has it in it p, then the engine will run rich.
The gas can come through a ruptured diaphragm inside and get sucked into the intake manifold.
If will start out slowly getting richer until, bam, it hits you that this has gone from yucky to terrible!

Check the connector on the AMM there might be a corroded pin or one that has gotten pushed back up inside. You cannot check enough things sometimes.
The engine will run or idle fairly decent with it unplugged (limp mode) but it pretty hard to drive off with it that way.

The adjustment on the AMM is not going to change your problem, it is too fine to do much.

The O2 is just doing its job and giving a rich reading out to the ECU.

Your mechanic only did the first logical step and that is to clear any ignition woes with your parts wallet. This is standard operating procedure otherwise most will not work on it.

If it doesn't help anything now, collect your old parts for your own diagnosis later.
A spare FPR is always handy as they do go bad! I have a fairly shiny collection from JY's.

One might think I have a shoestring budget! I do, until I wear shoes out! (:-).
You know, they are not so easy to find AS car parts are! Maybe, I should stop looking for used strings! (:)

I have learned to test or compare, the new to the old and keep the good ole' used boys around to help fight the war again another day!

Phil