I think the very ‘linear’ proposal that Lucas came up with Vader=Anakin totally out-of-the-blue, in the Spring of '78 while writing (or re-writing) TESB, is the least probable scenario.
Yotsuya, Obi-Wan wasn’t lying in the scenario that I propose. His ‘funny face’ or hesitation is the due to the fact that he knew that Owen never told Luke that his father was murdered. In the alternate scenario (while ‘alternate’ I think it’s more probable than the orthodox version(s) we are told to believe), Annikin/Anakin was indeed killed by Vader, the twist would have been that Vader - or whoever Vader really was beneath the mask - was the actual father of Luke, or barring that, family member X. And let’s be honest: how could Ben know for a certainty that his friend Annikin (original spelling) was Luke’s father? All the story really needed was the presumption that he was. And from Ben’s point of view, with his assumption of Annikin’s role in Luke’s paternity, he would have no reason to believe that his former Jedi student was the actual biological father of Luke, whoever he was prior to becoming Vader.
The genesis of a ‘secret identity’ for Vader goes all the way back at least to December of 1975, in a story conference which Lucas held along with writer/novelist Alan Dean Foster, FX supervisor John Dykstra, and a few others (Kurtz of course knew). How David Prowse could have found out about this is anyone’s guess. However, Lucas realized in Jan of 1976 that Star Wars might only be realized as a single film, so he re-worked the script to make the movie read as a stand-alone. Thus, a bigger role for Tarkin, superseding that of Vader. And Vader - on a surface reading at least - is just Vader, and had no other name.
When SW became a hit, and actuall full-budget sequel movies became a real possibility, Lucas re-visisted the ‘secret identity’ aspect of Vader’s character.
edit: For all the talk of the orthodox version* of Star Wars (*i.e. where there are absolutely no familial connections among the main characters whatsoever) exhibiting a ‘larger universe’ until the familial retcons in the sequels ‘shrunk’ that universe, this version of Star Wars would have been a stand-alone movie that would have inspired ONE sequel, at best. (say, something in the vein of Splinter of the Mind’s Eye, except where Luke would have possibly killed Vader), and probably only ONE prequel.