I'm pretty sure the line as stated on set was indeed "Obi-Wan killed your father." It does have the benefit of meshing OK enough with Hamill's dialog and performance to at least make sense, but it was never intended to be anything other than a deliberate obfuscation, and that's never where any of it was ever going to go.
IIRC Hamill and James Earl Jones were the only cast members trusted with the actual script (and for good reason; Prowse is kind of notorious for letting things slip in interviews.) Though even then, Hamill was only told on the day, right before the cameras were about to roll, and of course Jones' work was done in isolation, after the fact.
Out of the crew, only Lucas, Kasdan, Kershner, & I think Kurtz were in the loop. They even kept it away from Kershner initially until they got to the point where he needed to know. They even kept it away from Alan Ladd at Fox, taking no chances that it'd get out!
Again though; Vader's arc had they kept going with how things were conceived in ANH would have been non-existent. He wasn't supposed to be that kind of character. Just a henchman that would have died at the end of the movie with the Death Star. Indeed the shot of his ship tumbling away was done late in production and was achieved mostly in the edit with no new footage of Prowse filmed.
Also remember that the decision to kill off Ben was made during filming when Lucas realised he'd just be having Guinness standing around, not doing much except goading Luke a little over the radio during the battle. So the intent would have been for Ben to still be around for the next movie, and he'd be the one training Luke instead of Yoda. Indeed that decision is what necessitated creating Yoda in the first place. Similarly, the end of the movie was intended to essentially be what we got in RotJ, with the rebels recruiting a tribe of furry natives of a forest moon to overthrow the Empire (albeit by training them to fly starfighters.) But when most of that got lopped off, Lucas was so enamoured by the Wookiees that he made one Han's first mate. Which again necessitated cutting them in half and calling them "Ewoks" when he actually got around to that part. The main take-away here is that storytelling is a process.
Except that when you read early interviews with Lucas, such as in the Rolling Stone article mentioned earlier--and available online--his own words tell the story that he didn't have a lot of what became the final movies we saw planned out--and he certainly didn't have episodes seven through nine planned even though at one point he said that he did.
Yup, that's exactly the mistake I was talking about. You're conflating "has a general idea of the broad strokes and major story beats" with "having the final script all typed up and ready." Again: storytelling is a process.
The thing of it is, we have all the original drafts and can read them ourselves. Everything he said is right there. You can see nascent forms of the whole OT and the PT buried in there. It's not really up for debate.
