1) I can't be sure that any MAF will work properly, so I am taking a chance on one that was recommended.
2) Troubleshooting without satisfactory answers leads me to try a new approach, i.e. "possible MAF under-reporting airflow", that wouldn't be necessary if there was data I could compare my tests to, so I am trying it.
3) No, my X5 isn't liable to burst into flames due to a MAF swap, but might be set afire if my wife catches me replacing
A) all vacuum lines,
B) intake boots,
C) fuel filter/pressure regulator/fuel pump,
D) O2 sensors,
E) SAP unit, etc.,
any of which might be the real cause of the lean codes, but which also has eluded my testing, without exhausting all other possibilities first.
4) After many years of working on cars, racing cars, and working on machinery in my job, I agree with you about reputable brands being better, but have found that you don't always get what you paid for, and sometimes trying a substitute works out. In this case, not having a supply of BMW spare parts handy, nor the money to throw at it just now, I think it is justifiable in trying the cheap part for my version of troubleshooting.

No justification of your decision needed. Was only input to consider, an opinion, and you know what they say about opinions.

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X5 4.6 2002 Black Sap, Black interior. 2013 X5M Melbourne Red, Bamboo interior
Dallas