Well, thats a good start.

When you say it was 11.8 volts, was that with the engine running? If so, ignore the rest and get an alternator.

The o2 sensor codes, did they all come back with a heater code? Sorry, just read, yes they did.
Those are going to have a fused power source somewhere, its time to get back to the diagrams/fuse assignments and figure out where the o2 sensor heater fuse is (or what it shares a fuse with)

You might need to get out of the OBD codes and into real BMW codes to get more transmission info.