What a great thread. I really enjoy these found part search threads, so much research and details for such little pieces = very cool.
I certainly think your explanations are very plausible, but I guess so are some of the others with nothing likely to ever get truly confirmed. The one thing that does strike me though, is that in the pic just posted, the ESB, PP and ROTJ all have a three hole borden. And as your extensive searches have produced nada so far on the three hole version, the fact they were able to get not one but three - and possibly more for other versions of the helmet - leads me to believe they could very well indeed have been modified or machined..