QUOTE
It APPEARS that the simple act of removing the original plist, quiting Mail, and then re-installing the original plist after starting Mail
I think the key word there is "appears".
If you, indeed, did exactly what you say above, Mail is still not reading the old plist. Programs read their plists when they first start and may even copy the settings to RAM, if they are so programmed. But if you started up Mail while the old plist was not in its expected location, it simply created a new one and is now using that one.
OTOH, if, when putting the old plist back in the proper location Mail was not running and you got the normal, "There is already a file with that name..." dialog, Mail will not know anything about the moves and will use whatever plist is where it looks for it.
I would be extremely surprised at any program that didn't at least complain, if not downright crash, if one were to swap plists in mid stream, so to speak. Perhaps Apple has planned for that kind of user misdeed?
In fact there is actually no need to actually move any plist files, simply renaming them serves the same purpose, each program is hard-wired to look for its own, specifically named plist. It has no idea that an exact copy, except for the edited name, may exist and be usable.
That is one reason I detest people telling others to "Trash" a plist. Many new users equate that with "emptying the trash". When that is done, and the problem was not related to a corrupt plist, you have now forced that user to completely rebuild all his preferences, many of which he may have forgot he even edited.
And the original problem will still exist.
