Wow, Z&B, thanks for all that.
I'm frankly astounded that the 1984 Guide dates the formation of the Empire to only 18Y BBY. I really can't fathom a sensible reason why this would be, except that Lucasfilm figures Darth's turn must have coincided with its formation so that he could be good before and evil after. However, it would make a lot MORE sense if he became evil while the Empire was in full swing--after all, wouldn't that be when going dark would be the "easier, quicker path," i.e., the path of least resistance? Seems that to turn evil in a just Republic and to then go on to participate in the transformation of the Republic is more ambitious and driven than it is "quicker and easier".
Yet if the "Republic falls" some indeterminate time before the "Empire forms", I suppose that means there was some intervening period of the "New Order"? I guess that some proto-Empire fascist order might serve as a good political milieu in which it would be personally expedient for Little Orphan Ani to break bad.