Well... Turns out that yet again a MAF code DOES NOT mean a bad MAF (unless I got a defective one...): turns out the code is back so I had to do some digging.
So first of all... Here's what the MAF is used for in our diesel (all sources are BMW litterature): "The HFM 6.4 measures the air mass intake within very close tolerances so as to permit precise control of the exhaust gas recirculation as well as optimum configuration of the smoke limit. This is important for complying with current and future emission limits." and "The HFM 6 hot-film air mass meter is located downstream of the intake silencer and is fitted directly to its cover. The HFM measures the air mass taken in by the engine. This is used to record the actual air mass, which in turn is used to calculate the exhaust gas recirculation rate and the fuel limit volume. There is also an intake air temperature sensor located in the HFM housing. The temperature is evaluated by the HFM and sent to the DDE as a PWM signal." So, different role than a gasoline engine. Or at least it's used to affect AFR in a much less direct way...
Now with that being said the code 3FF1 I get is a failure to plausibility check: "the ratio of the calculated air mass to the measured air mass violates limit values that are dependent on the operating point". What that means is that the DDE calculates based on a series of parameters that air muss should be A but at the MAF is measures X*A and X is outside of a certain allowable range. Since I just replaced the geezless sensor I assume the measured value is correct so the problem is the calculated value.
A quick interject: the plausibility check is done occasionally under certain driving condition call it "cruising condition" which is "no sudden changes in torque demands"..
Now I have no freeken clue how the DDE calculates the air mass value but it's probably a condition of MAP reading, Wideband O2, EGR duty cycle and a bunch of other things, including, according to BMW litterature, the charge temperature sensor (i.e. the temperature sensor mounted in the charge pipe on the driver side, right before the elbow leading to the intake manifold): "The charge-air temperature sensor records the temperature of the compressed fresh air. It is located in the boost-pressure pipe, directly upstream of the throttle valve. The charge-air temperature is used as a substitute value for calculating the air mass. This is used to check the plausibility of the value of the HFM. If the HFM fails, the substitute value is used to calculate the fuel flow measurement and the EGR rate."
In essence there's a temperature sensor built into the HFM (a fancy name of the MAF) and another one on the charge side...
In any case. I removed the charge temperature sensor. It was lightly coated with oil so I gave it a clean, hopefully that was it (i.e. the oil coating was causing incorrect temperature readings... could be...)... Time will tell...
Just chronicling my adventure but who knows that someone out there have gone through similar endeavor