Thanks for the reply, AMcG. All of what you say makes sense, so maybe the focal plaque formation is a bit random. The weakness in the vessel wall appens most significantly farther upstream, and that is where the breakdown of the BBB leads to formation of a plaque. I have not really incorporated the deposit of iron into my thinking. I am still thinking along the inflamation/immune response eitology, and that may not be accurate.
Posibly the vessel wall is strongest downstream, so it is most likely to be damaged farther upstream, i.e. deeper in the brain.
I am not a doctor, either, but it is fun to try to think this stuff through. Plus, it is not as though the doctors have figured out what causes MS yet either, and they have been working at it over a hundred years.