[BigHanksCustoms]

For me the one thing that Obi wan says that is telling is when he says (paraphrasing) : "A young Jedi named Darth vader, betrayed and killed your father..."

If there was an established knowledge of the Jedi, even a cloudy "mythological" knowledge would they not also have at least the same minimal knowledge of the Sith?

And having said that would Luke not know that Darth is not a name but a Sith title? How would Obi Wan be training a Young Jedi who is a titled Sith?

If the "A New Hope" trilogy is viewed as a standalone, I can buy that "Darth Vader" would be the name of a person, not a titled Sith. Why Aniken chose that name (in the context of the first trilogy only) would be anyone's guess though.