Hoo-Boy, wish I had just ONE customer recite auto history as completely. I'd usually have to imagine what their washing machine sounded like on spin, then apply that to the car.
I have to agree with CPOM, dealerships need revenue due to high overhead. Very few have good repair personel, dispite having superior diagnostic equipment and literature.
I am also in agreement with the intake gasket being the main culprit, and setting the codes. Even the SMALLEST vacuum hose leaking will eventually send the O2 sensor to the rubber room. Figure open throttle, max'd TPS signal, O2 sensor's brows raised, all hands on deck in the ECM altering timing and monotoring engine temp and RPM etc. Then along comes extra air from the manifold leak, the ECM has already adjusted for max everything so can only fail what it can't compute.
I'd suggest changing the intake gaskets, it's the cheapest item on your list, and will probably bring the best result. If not, post again. There's a lot of bright minds on this forum willing to help...Cal