See, I don't think there's anything wrong with knowing Luke is Anakin's son if you can dissociate Darth Vader from Anakin. If we think he dies at the end, rather than having us see him become the Darth Vader from the OT, that may be all it takes. As for why Ben would say that Vader killed Anakin in ANH, we might think it's to cover up the fact that he killed him.