I think it's worth noting two beings, one currently out of canon, the other in it. Jolee Bindo, in Knights of the Old Republic was a "Gray Jedi." He was definitely a good guy, and he used to be a Jedi, but he rejected the strictures of the order.
In Star Wars Rebels (which, unlike KOTOR, is currently canon) we have the Bendu, a powerful force sensitive entity, which is neither of the dark nor of the light. The Bendu is wise and powerful. But he's not purely "good" or "evil."
So we've had characters that rejected the binary before. And I suspect this will be dealt with more in the future.
However, I don't see Kylo Ren being redeemed and getting a peaceful end. He is a mass murderer.
Very interesting points! But he was a mass murderer before Rey offered him redemption, so I still am holding out hope for a better ending than Vader!
Rey is a sweet summer child. And I don't think she fully realizes (or realized, anyway) the breadth of his monstrosity. She wasn't with Poe in the village when he gave the order to kill everyone.