It must have something to do with the Void Century. Roger wanted Oden so they would be able to read the poneglyphs, so he definitely knew something was up. Rocks was also said to be studying the forbidden history, so the rest of his crew should know about it. The Ohara scholars knew the name of the ancient kingdom from their research, so I'm guessing that all of the major players assume that the ancient kingdom is the final island, and they probably think it will give them access to the ancient weapons.
Maybe nobody knew about the existence of the One Piece but they knew about the final island at least.
As for the last part, I really have no idea. I haven't the slightest clue as to what the One Piece could be. All we know is it made Roger laugh hysterically. I dunno, I don't think some weak sauce pirate finding it would change anything.