An interesting hypothesis, Chnnel72.
If it helps you further, with dates, Lucas is on record in August 1977:
‘August 1977, after the release of #StarWars, Lucas stated that Vader massacred the Jedi but Kenobi and Luke’s father Annikin survived’ - https://twitter.com/philszostak/status/1161026993917317120 (Lucasfilm creative art manager & author)
(But he does also say: ‘Darth Vader wasn’t Luke’s father until the second draft of #EmpireStrikesBack in April 1978.’)
And according to Marcia Lucas in the recent “Icons Unearthed” documentary:
'The third episode of the docuseries is all about the making of The Empire Strikes Back. George Lucas farmed out the screenplay to science fiction novelist Leigh Brackett, but Brackett passed away after she handed in her first draft. As has been documented in many different sources, Lucas then collaborated with Lawrence Kasdan to rewrite Empire, and it was during this process that the big twist about Vader was invented.
As described by Marica Lucas in Icons Unearthed, George Lucas was struggling to come up with some kind of big twist for Empire’s third act. While stuck, Marcia and George invited screenwriters Willard Huyck and Gloria Katz over for dinner. The couple had worked with George Lucas before, including an uncredited rewrite of A New Hope. When Lucas described his struggles with the Empire story, Huyck joked, “You can always make Darth Vader Luke’s father.”
In her new interviews, Marica Lucas says of this moment, “I kid you not. I don’t know if Willard even remembers saying that. But I remember it.”
According to Marcia Lucas, this offhand comment — even though it was a joke — stuck in the mind of George Lucas, who then, of course, transformed the entire Star Wars saga. Suddenly, retroactively, the whole story of Star Wars became about the Skywalker family and the tragedy of Darth Vader.
There’s no evidence to suggest Marcia Lucas is wrong, either. Brackett’s draft of Empire makes no mention of the secret that Darth Vader was a Skywalker who fathered Luke and Leia. Mark Hamill has also said more than once that the twist of Leia being Luke’s sister was a very late-in-the-game idea.
Marcia Lucas’ revelation that the Darth Vader parentage idea may have come from one of George’s most trusted friends is fascinating. Gloria Katz and Willard Huyck went on to write Howard the Duck and Indiana Jones and the Temple of Doom, neither of which are considered the best that Lucasfilm had to offer. And yet, if Willard Huyck did invent the story of Anakin Skywalker, then suddenly he becomes the most pivotal writer in the entire history of Lucasfilm.’
from https://www.inverse.com/entertainment/darth-vader-luke-father-joke-marcia-lucas