BMW Repair: emissions, oxygen sensors, threshold bank

Question
I am a new owner of a 2003 M3 with 42K miles.  It failed NY emissions due to check engine light on. Inspection report diagnostic calls out P0172 System Too Rich Bank 1 and P0430 Catalytic System Efficiency Below Threshold Bank 2. I had a second diagnostic report by my mechanic and it pointed to the catalytic converter ( which is still under warranty), Bank 1 Converter is damaged. Upon taking this to BMW, their diagnostics is saying all 4 oxygen sensors are dead (not working) and need to be replaced at a huge cost. Furthermore they say they can not give me their diagnostic report to take to my mechanic because of proprietary info I'm told.  Can 4 sensors go out like this?  Is this likely the problem? Upon asking the BMW rep if he has ever seen this he said no. Obliviously the sensors are not cover under warranty any longer. They also say that they can possibly reprogram recalibrate them.  Is this true and if so wouldn't they be working?  I am not a mechanic, especially of a car so technologically full- but I feel uneasy about some of my conversations with this BMW dealership.

Answer
Very, very, very few BMW dealerships/mechanics are unreputable.  There's just too much to risk for a few extra dollars.

Yes, all the sensors can go bad.  At once?  probably only if another problem.

The '03 M3 is complex enough that you are going to have to trust a dealer.