About the obi-wan never beeing found, I've heard this theory (*could be a spoiler if I am right*) that Obi-Wan fought with Anakin after he became Vader, and Vader won, but he saw Luke with Obi-Wan, did not kill Obi-Wan, and said to both of them to run away and never to come back. That would explain the following things: "You should not have come back", Obi-Wan hiding Luke on Tatooine with Lars under the name Skywalker, Obi-Wan knowing Vader is Anakin, Vader knowing Luke is his son but not knowing about Leia (which was hid under the name Organa).