The big script thing Lucas has peddled isn’t true. The original Star Wars sequel was called Star Wars II by Leigh Brackett, and Darth Vader wasn’t Luke’s father.
In fact if you read the story conference for Splinter of the Mind’s eye, everything we’ve been told about the Saga of Darth Vader goes out the window.
The script from 1974 Vader was just some general, and Valorum was the Sith Knight. Luke was an old General.
I’m glad at least the secret history of Star Wars is out there for people to read who were only given a certain point of view.
Have you read all the drafts of Star Wars? That one was one of the earliest. As the drafts went on, the story changed and it got closer to what we see. For the film we got in 1977, he cut out the middle. How do we know this? Because some of that was sketched and drawn. The evolution of the Milliennium Falcon has all the pieces of this. Originally they did not arrive on the Death Star, but on Bespin, which had a different name and was an imperial outpost (or capital) where Leia was being held. The original story had more in this section and then the whole Wookie battle while the death star battle was going on. That part of what Lucas said is certain. The Bespin floating city idea wound up in TESB and the forest battle with primitives (now Ewoks - wok-e to e-wok and 7 foot to 3 foot) ended up in ROTJ. The rest of the story evolved and changed, but Lucas is absolutely correct that he cut out a bunch that he recycled in the other two OT movies.
As to when Vader became Luke’s father, we don’t know. I’m sure it was not in the story treatment that Lucas gave Brackett to write the screenplay. It is very clearly not in what she wrote, but the reveal scene is. We don’t know if Lucas was being secretive or came up with the idea later. We do know that before the novelization was finished that the writer knew, Lucas and Kershner knew it because they told Hamill. James Earl Jones and Ben Burtt had to know (to record Vader’s lines and add the FX. So it was well planned in advance. One later copy of the script that was digitized just has notes for the reveal scene indicating that something was secret about it at that point. Kasdan probably knew as he probably wrote it… unless George wrote that part himself.
So it is in error that Lucas pulled it out of his ass at the last minute. There are plenty of indications that he had it planned for a while before filming. Who knows what he told Alan Dean Foster. Who knows if what he told him is what he was thinking. That is a mystery. A lack of evidence of one thing is not evidence of something else.
Lucas has become a master of rewriting history, but most of the time he takes a shred of truth and embellishes it. I have no doubt that in his mind as he worked out the story of TESB that it came to him at some point and he has projected it back into the older drafts where there is no trace of the evil general being the hero’s father. But there are hints. In one draft the father is a cyborg. So I have no doubt in Lucas’s mind that is the origin of the idea. Plus we only have some of his written drafts. It is by no means a complete picture and does not tell us all his thoughts and early rejected ideas that later came back to him and he incorporated. There are echoes of the fallen father and twins in earlier drafts, but no where close to what we got. Enough to say Lucas isn’t outright lying, but not enough to say he is accurately remembering what he wrote in those early drafts.
Also, he evidently reread some of those old drafts before he wrote the PT as a great many unused things in them made their way into the PT.