Kinda sorta. The word "Sith" has been around for a very long time. Darth Vader was referred to by promo material as "Dark Lord of the Sith" (though never onscreen). Of course for the longest time, nobody knew what the hell that meant, or what exactly a "sith" was (including, I think, Lucas himself).
No. They existed in Lucas' notes for the original movies and backstory, Vader was called the Dark Lord of the Sith, and the Tales of the Jedi comics (which were the first thing to really explore the Sith) pre-date the Prequels by around 5 years or more.
I've had the misfortune of reading some of KJA's work. He is, without a doubt, the worst professional author I've ever read. Bar none. I don't think any criticism aimed at Anderson could possibly be exagerated.