Those Making Of stories have long been shown to be Lucas revision many times over. No different than his attempts to revise history in other written forms, documentaries, and interviews.
Yes, we’ve known about the latter-day interpolations in that book. But if you take the quote about midi’s at face value, all it means it that he had the concept of midi-chlorian back then in 77, but he wasn’t using that word for it at the time. And he said that they were found in certain alien species, not humans (and certainly wasn’t in reference to any ‘chosen one’).
The excerpt from that Dec 1975 conference is not a revisionist interpolation. Note that he says that Luke gets the girl, instead of Han. He says that Han leaves at the end of the second book, not gets frozen and captured by a bounty hunter…and note that the ‘leaving’ subplot is taken up again once he and Leigh Brackett start writing the sequel in late 77. Also we find out who Darth Vader at the end of the second book, which merely suggests a secret identity under the mask. If this was a Lucas revisionist bit, he would have said, “we find out that Vader is Luke’s father/Annikin Starkiller(Skywalker)”. Something to consider.
Lucas wouldn’t have been discussing a second sequel two years before the original film even had a finished script. It didn’t even have a finished story at that point.
December 1975 is not two years before the finished script. The script was finished in March 1976 (the 15th I believe). That was about three months away. What intervened in those months - during January to be precise - were the budget cuts demanded by Fox as a condition of the go-ahead/green-lighting of production. In making these cuts, Lucas had to come to terms with reality and make the story/film work as a stand-alone picture, should sequels not get made. He was forced to think pragmatically. That is where the idea of killing off Ben came in. Anyway, he has said that prior to that, in the period between drafts three and four, he deliberately came up with ideas as to what happened before the current story (SW), and what happened after (sequels). Despite your objection, he was considering two sequels during that time (Dec 75).