I think it’s flawed thinking to say that because Luke did the right thing at the end of ROTJ, he’s automatically immune to temptation for the rest of his life. The message is that even heroes can fuck up. And Luke ultimately doesn’t even harm Kylo, he just stalls him, leaving the door open for any sort of reversal in IX. As for Leia giving up on him, compassion was always Luke’s deal, not hers, so I really don’t think that breaks with anything. Leia’s priorities have always placed the highest importance on the galaxy at large. TLJ definitely throws some stuff into question, but I really don’t think it makes any universe-shattering conclusions.