In my scenario, the viewer doesn't find out about Darth Vader being Luke's father until ESB either. The multiple masked Sith Lords lead the viewer to believe that Darth Vader was always one of them and Anakin is a different person (since we see him duel one of the masked Sith). The concept of a Sith Lord taking the name of the first Jedi he kills is very interesting though. Maybe that could be worked into my synopsis as well.

In my synopsis, I did establish that Obi-Wan had had other pupils, and the viewer would find out that one turned to the Dark Side. In Episode IV, the viewer would just assume that Darth Vader was in fact that former apprentice, though in reality there was another as well.

An additional point: Obi-Wan would adopt the name "Ben" once the Empire started hunting down Jedi, to make his statement in ANH make sense.