Take a look here on how to test and repair the DEF sensor: X5 35d CEL P205b - Page 4 - Bimmerfest - BMW Forums. What is described there does not deactivate the sensor in any way. It simply fix the self diagnostic circuitry of the sensor which is prone to failure.I would start with that.

Regarding the other code, here's a couple of thoughts:
1) Take a look at this: Reducing agent metering valve actuation fault. - Bimmerfest - BMW Forums. It describe how to diagnose the metering unit which too can fail. May not be your problem, but removing, testing and cleaning the metering unit could help.
2) The NOx sensor module is located right below the drain of the A/C. I believe I read someone who was having issues with the NOx sensor and the reason was corrosion on the actual module, not the sensor itself. Take a look here: A/C drain location - Bimmerfest - BMW Forums. That's what I've done on mine after noting corrosion starting to appear on the module and the crossmember. May be worth a look for you and perhaps remove the connectors and clean them with contact cleaner. But it may be completely unrelated.

Oh, I and others would really appreciate it if you could report back the results...

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