I had the exact same problem with my MAF on a '01 ML 430. I sprayed brake cleaner into the square opening that you referred to:

"At the end of the sensor insert, there were some small square openings and there appeared to be aditional metal objects inside".

That is the wire you need to clean. From my limited understanding of how the MAF works, if dust collects on this wire it will impede the cooling effect of the air stream and throw the sensor off.

Initially I was disappointed with this approach because the CEL remained on. However, my understanding is the computer needs a couple of 'error-free' starts before it resets. 3 or 4 starts later it went out and hasn't come on since (only 3 days and counting). So I will keep my fingers crossed for the time being.

Good luck.