Nobody knows Sutpen's entire story. The narratives we read in Absalom, Absalom! – whether second-hand or fourth-hand – are all very partial and flawed through the influence of sorrow, bitterness, regret and revenge. The notion of the past is tied up with the crises that the various characters experienced. The past bears down very strongly on the present, and no one can escape Sutpen's influence. In fact, the narrators' version of events is more about misremembering and fantasy than recounting facts. All of the characters are at once overly intimate with their own pasts and completely incapable of representing reality. Sound complicated? That's because it is.
The entire story is based on people's memories. As "living" as Sutpen is to the reader, we never actually seem him in real time.
Miss Rosa would have been a lot happier if she didn't spend so much time dwelling on the past.