What will the crew of Battlestar Gallactica find when they get to Earth?

Some thoughts on Season 4, the final season, of "Battlestar Gallactica," as we have just begun watching: what to make of the reappearance of Starbuck, in a fighter plane in brand-new condition and believing she's been gone a few hours with the Gallactica says it's been several months and she was presumed dead? She says she's found Earth (and her descriptions of Saturn, for example, suggest that she has). Possibilities: there is an obvious time warp separating BSG from Earth, which she must have passed through; part of the question isn't whether she and her crewmates will find Earth but what will they find when they get there? A future civilization? A past one? A place that they themselves had left in an earlier millenniuum? Perhaps it's an earlier civilization and the Caprica survivors build things and leave signs for the future: e.g., the Pyramids (as in their national sport)? Second: who is the 5th Cylon, and why did this season begin with a flashback episode about a group of Cylon-human hybrids under development? Is Starbuck a hybrid? What role will they play if any (or was it just a random episode that didn't fit anywhere and that they popped in between seasons 3 and 4?