The decoy Vader thing is really a case of Your Mileage May Vary. I think going for it just creates too many hoops one would have to jump through in the writing process, and just accept that Anakin and Vader being the same is just common knowledge. Same thing with trying to shroud Yoda in mystery so the audience doesn't immediately recognize him when he appears in ESB.

I do think that if done well, you could explain Obi-Wan's statement about a "certain point of view" and his lie to Luke is a reflection of his own guilt regarding what happened. While most would see Anakin Skywalker becoming Darth Vader, maybe Kenobi believes that Vader in some ways was emerging even before the worst aspects of Palpatine's seduction, and that he simply failed to see that there were almost two students in one individual both fighting for control.

One was the same idealistic and good hearted young man on Tatooine devoted to his friendship and learning with Kenobi, while the other was a bitter, cynical, unforgiving, and opportunistic pragmatist out who doesn't see the Jedi and possibly the nature of the Republic as things utterly crucial to the future of the galaxy.

Thus he really did think of it as Vader killing Skywalker, and that Obi-Wan's mistake led to Anakin's destruction and Vader's emergence. By the time the two fight at the end of the prequels it becomes sadly clear to Kenobi that Anakin was dying before the fight ever started. Its Vader that he's facing, its Vader who really took control the moment Skywalker compromised himself to Palpatine.