You could offer that sort of explanation, but it's not going to change the fact that when Obi-Wan says his bit in ANH, the viewer is going to assume it means Vader is Anakin, regardless of how many other interpretations are technically possible. We want to see this from the point of view of a new viewer, not one who handwaves things away.

The only way out of it that I can see is to make ROTS such that Vader is in it, but is not mentioned to be Anakin, and the relationship between them and Obi-Wan makes it plausible that he can shift the blame for Anakin's death onto Vader in ANH. Obi-Wan must appear to kill Anakin on Mustafar, or at least leave him to die. Vader should then show up in his suit and attack the Temple — the Old Republic MMO trailer has what I'm talking about in terms of imagery — or do something else evil that Obi can react to. Even this, though, will be difficult.