Good stuff, Bob. I've always been interested in plug design and behaviour because it causes me so much grief in our fleet of vehicles!
From what I've read, including some good stuff on the NGK website, one reason for the multiple ground electrodes is to reduce electrode erosion because the spark is shared between 2, 3, or 4 electrodes. (Only one electrode sparks per firing stroke.) I see this design is still used in aviation plugs.
I suspect aviation engines have to deal with accelerated erosion because they normally run in the medium to high load range, especially under wartime conditions. These loads require spark voltages in the upper range, maybe 10kv to 20kv, hence more wear.
Maybe that's why the switch to single electrode plugs on tank engines as the load is much more variable so average spark voltage would be less, so less erosion. I wonder if they were hotter plugs too, as i believe fouling was a problem due to longer periods idling than these engines were designed for.
Malcolm
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