Linc, Thanks for this, it's not the first time you have been of indirect help to me and many others I assume.
This one tip was needed to get rid of a very nasty and persistent state of Mail, after a series of beta Yosemite previews and consequent updates. At one given moment the mail app must have had a tiny thingie nuked leaving it in an unusable state. No addition of an account was possible, not accounts that were set up in SysPrefs / Internet accounts were being loaded.
I now have my main bug resolved. Not really a bug, as another user account on the same machine had no problems, so I considered this to be a state of bad luck for the given account.