“Just on a basic level though, why does Kylo’s redemption have to be a part of Rey’s story?”
I don’t know how else to answer this for you! LOL. By the point his redemption becomes legitimately viable, it’s firmly her story and he’s HER villain. He’s not a supporting or secondary protagonist. I don’t understand how you possibly make his redemption NOT in service to her story under those circumstances, and those ARE the circumstances by the time his redemption is seriously on the table. It’s not a what-if or a hypothetical at that point. It’s 2 1/2 movies into a 3 movie cycle that is absolutely her story, and his place IN that story is just as firm and absolute. He’s the villain of her story. His redemption needs to be in service to that to be successful.
And even if I grant the argument his redemption as it is serves her story, just the mere fact it serves the story isn’t enough to overcome how poorly it’s done. Just because a thing is baseline done doesn’t mean the doing of it was laudable. The Prequels tried to do a lot of things, and you can argue that those things WERE achieved. But they were largely done poorly. The execution of a storytelling goal is just as important as the arriving at it.
I disagree that Kylo’s potential descent into irretrievable villainy for the third movie is “in direct contradiction” with what came before, but we’ve done that do-si-do, LOL. Kylo Ren’s direction as a character is up in the air after his defeat on Crait. The film itself makes an argument that he could double down on pursuing the dark just as clearly as it makes the case Rey could have turned him. I don’t think a failure to be redeemed in part 3 contradicts anything that’s there in the first two Sequel Trilogy stories. I feel if it directly contradicts anything, it’s that conventional wisdom of “Star Wars” as retconned by Lucas via promoting Anakin’s redemption story as its ultimate “true meaning”. But like I’ve said, I find that conventional wisdom not only to be fundamentally broken, but unnecessarily limiting.