Thank you. You probably right, either way.
No prob! It looks like we were both kinda right. Lucas did indeed “split the story” and “stole the ending from the second half” …but he did both of these in the space of a single draft (the second full draft), so all the full drafts indeed end with a battle with the empire’s new battle station!
Honestly just leaving it to “Lucas made Vader Luke’s father because he wanted to go back and depict the cyborg father’s heroic sacrifice and the black knight’s turn to goodness from the original scripts” is actually a lot simpler to digest than having to explain the whole “expanded-then-recondensed-story” rigamarole, but that’s the process that got me to figure it out myself.
From “THE DEVELOPMENT OF STAR WARS AS SEEN THROUGH THE SCRIPTS BY GEORGE LUCAS” as archived at https://www.starwarz.com/starkiller/the-development-of-star-wars-as-seen-through-the-scripts-by-george-lucas/
Lucas had realized that his first screenplay would not fit into one movie, so he put a large part of the rough draft aside when writing the second. Since he now had material for three films, he decided that he would use the deleted parts if he ever got the opportunity to do any sequels.
This second draft took Lucas eight months to write. He had pared the previous screenplay, basically cutting it in half, but it still contained two movies; the rescue mission, and the battle of the Death Star: “I sort of tacked the air battle on, because it was the original impetus of the whole project.”