I feel the same way with trying to avoid replacing good parts. Although, I only spent about $45 each for the Bosche O2 sensors. They are much less than the OE ones ($130?)and are identical except won't have a BMW logo. BMW uses Bosche O2 sensors as OE. Because both the O2 sensor and the MAF degrade slowly over time either can be causing your problem. More importantly, even if you detemine which is the culprit the other may cause you grief months later. Do you have any repair records? I would check with that dealer and see if they have any records for the replacement of those parts.
When the MAF is unplugged the computer will revert back to a 'standard' fuel map. I'm not sure but would guess that when everything is functioning there should be little change at idle so O2 sensor values should not change a whole lot. I'm curious so I'll try this tonight and let you know what I get. I have a non BMW scanner that works with a laptop which gives me the O2 fuel trims, not sure if it will give me the voltages but that will still help figure out what should happen with/without the MAF.