Very cool stuff! I love studying the history of SW and the “when was Vader made Luke’s father” debate is one of my favorite topics to research. I’ve done a lot of analysis on the topic (based off of both the discoveries of other people, a few discoveries I’ve made myself, and the confirmed documented SW history like the Lucasfilm books and the public older drafts) and as of now I personally believe that Lucas is genuinely telling the complete truth that he had the Vader=Anakin twist fully conceived in its entirety by the third draft of ANH and just briefly considered scrapping it during ESB’s writing. I’ve gathered a lot of evidence to back this up, which I’d be happy to share with you guys if you’re interested.

I highly doubt Marcia’s claim is real. In that same interview that she said that, she said so many other things about SW’s history and her role in stuff that’s just blatantly not true; An example includes her saying “I never even knew there were two stories” which is obvious nonsense because the second ANH draft ends with a sequel teaser, Lucas said to Foster in December 1975 he planned two sequels and a prequel, and Mark Hamill said the actors were all signed on for a trilogy and Lucas mentioned an Episode IX to him in 1976, and I highly doubt George wouldn’t have told his own wife and editor about these plans especially since he already asked her for help with story related stuff multiple times before. Other examples include Marcia saying she suggested to Lucas to cut the Luke and Biggs scenes when Rinzler’s book says Marcia fought to keep them in, Marcia saying the idea of killing Obi-Wan was entirely hers when George said in a 1977 interview (which is years before the divorce so there’s no bias there) that he was already considering the idea of killing Obi-Wan at the time Marcia suggested it, and more. So for now I’m taking Marcia’s claim with a grain of salt because it doesn’t seem like Marcia’s recollection of SW’s history in general is very accurate or unbiased.

@Tosche Station, I’ve read through your posts on both here and the Jedi Council forums on this topic, and I just want to say I really admire your research and theories; you’ve come up with some fantastic and really well ideas and takes on this topic that I never even thought of. I would like to know though where this “google search link” where you found this alleged Kurtz quote is located; I copied and pasted the contents of your quote into Google multiple times and nothing came up. It sounds very intriguing and I’m curious to see it myself. But even if it’s real, it seems to make very little sense to me for two reasons:

The idea that Lucas had Vader being Anakin planned in the third draft, but then completely dropped it by the fourth draft, is pretty hard to believe because as far as I know, almost nothing about Vader and Anakin actually changed between the third and fourth draft. The only thing that changed was mention of Luke being raised by Anakin as a young kid being removed, but we know that was still Lucas’s intention even after it was removed from the script because Lucas told Brackett in the ESB conferences that Anakin was the one who brought Luke to Owen and Lucas also said in an interview in 1979 “The prequels are about the early life of Luke’s father when Luke was a little boy”. We’ve also got things you mentioned like Anakin being a pilot and being killed by Vader being mentioned in the fourth draft for the first time, but I think that’s likely just a result of Lucas deciding Luke shouldn’t know his father is a Jedi and Ben telling Luke about him, which was decided by a random note as shown in Rinzler’s Making of Star Wars book.
I found a video showing a clip where Kurtz is interviewed where he mentions that he doesn’t believe Vader was always intended as Luke’s father, and he mentions nothing about a “Anakin killed Vader” version of the twist at all. It’s shown in the youtube video “how to watch Star Wars: the George Lucas sequel trilogy” by Rick Worley at 1:40:51