It's puzzling that the MIL light still comes on periodically to indicate the same fault. But the time period between these occurrences is now usually very long and so not as concerning. Sometimes it clears itself and other times I clear it out of curiosity to see how long it will take to reoccur. For example I drove 200+ miles of mixed urban and motorway driving before it relit. It's like I've turned the clock back to when the problem first started.
So to me this indicates the cause has been an accumulation of problems with one still left to be identified. In retrospect I think the major culprit has been the EGR valve not the MAF. Most probably cleaning with specialist MAF cleaner would have been sufficient.
Last time the MIL illuminated was under heavy acceleration. Is this a piece of information that could indicate the cause?
