How the Jedi know about Bane
ok, so I think that the evidence points to the fact that the battle of Ruusan wasn't the sith extinction one thousand years before episode I. It is mentioned in both the movie and novelization and in the novelization of episode III, Yoda mentions Darth Bane by name and there are several references to the Sith ruling the republic at one point. I think that conjecturally, this means that Bane and his apprentice layed low and eventually began manipulating the chancellor until they finally took over the repubic. The Jedi found out about this and killed both of them, learning all about Bane's new laws of the sith. I supposed Bane's student took on an apprentice at some point, violating the law of two but preserving the sith legacy. This apprentice was either the first in a long line of hidden sith or could have been Darth Plagueis himself (using his power over midichlorians to keep himself alive for as many as 800 years). It's also possible that the jedi thought they had killed the apprentice, but the apprentice used the force to heal himself, meaning that Bane's direct apprentice was Plagueis. Either way, this would explain the Jedi's knowledge of the Sith laws of Darth Bane and the references in the episode III book.