Catalytic Conveter Diagnosis HELP!

[Fortress]

I've read extensively through the forums but would greatly appreciate some additional input.

The story: I have an '06 X5 3.0. 117k miles. Check engine light came on and I pulled codes P1093 pre cat fuel trim too rich bank2, and P0430, cat converter conversion 2.
Car runs fine, no power issues nor running issues, no sulfur smell, etc. BMW Dealer (Bridgeport, CT) said they were "certain" that it was a Mass Air Flow Sensor... I replaced the MAF, still get check engine light, and same fault codes. Then I replaced the secondary MAF. Same deal, CEL/codes came back. Dealer refused to make good on the bad info... This was initially a PPI, $225, which turned out to be a huge waste of time and money. They missed a TON of stuff and had me chase non existent MAF issues. PS, BMW NA says "they have no control over the service of their dealers." What?!!!

Brought it to a reputable Indi. and they said I had bad 02 sensor, bank2, sensor1 (pre cat). Replaced that $279-(what?!) and still have same codes and CEL. Brought it back, (they were annoyed to see me again), and they said the other 02 sensors are functioning properly. They say its a bad rear Cat. They swapped the rear 02's as a test, and said the wavelengths registering point to a bad rear cat.

I cleared the codes and the car ran for 300 miles w no CEL. I figured I lucked out and escaped a cat replacement. I occasionally get a brief burning smell. (not sulfur) Thoughts? The Indi said all of this was likely from the previous owner burning up a coil that blew the rear cat???

BMW dealer says rear Cat is an 8 HOUR job... That sounds crazy.. Another shop said 3 hours. Another shop said an hour...

Question to any QUALIFIED folks out there is do I replace the 2 rear 02's, and see what happens? Full beans and replace the cat? Or do a smoke test on the vac system, (which the Indi said was a waste of time...?

Any DIY on replacing that rear cat? I saw a previous post that said "remove and replace"... That won't cut it for an explanation. Anyone with a Bentley Manual out there? I see a fairly clear path to get at the rear cat, but the manifold end might be a PITA.

Any advice would be greatly appreciated. I'm tired of paying shops $200 at a clip for guessing, and simply throwing parts at this.

Thanks folks!