If an edit was to be attempted that gave Obi-Wan a more explicit reason NOT to kill Vader; could Obi-Wan sensing Luke in danger not be used? Bring Obi-Wan’s visions of Luke’s peril further forward, so that he senses this soon after Vader is defeated and so rushes away more immediately after their duel? It would probably entail losing the “So long, Darth” line - which is shame!
But the whole drama with Luke is just so nonsensical and, arguably, canon-breaking. And Reva’s redemption for having spared Luke is unearned. None of it works, and I feel like it’s all more offensively bad than Obi-wan sparing Vader’s life—especially since it is conceivable that Obi-Wan would spare his life.