I have a whole other post I did about the original Star Wars trilogy that is relevant to this, but I'll try condense it:
So one, yes, absolutely it is entirely correct to take issue with Darth Vader's apparent forgiveness by the Force, there is no need for Leia to accept him as her father or to feel anything for him besides hate and contempt, redemption takes more than turning back for a couple of hours and then getting out of culpability by dying, all fair.
That said: the original trilogy is Luke Skywalker's story and the story of Luke Skywalker, on a meta level, is about being a young adult in the 60s and 70s who did not experience WWII or the depths of fascism personally, but who grew up with gaping familial wounds - family members who you never knew but who older people refer to or talk around, people they compare you to, figures who other children had in their lives but you didn't. Someone who as a child was given fantasies of heroes fighting daring battles, who was told it was all about nationhood and fighting for your people and the course of civilization, someone who internalized those principles as guiding lights for their own morality and who they want to be... and THEN finding out when you become an adult, and are permitted to know about the horrors, that it is not just honor and glory in your heritage, that you, 70s white boy, may have evil and darkness and the corruption of all your values as a potential to fall into just as your father did, the temptation to hate and cruelty and domination and atrocity. And the absences in your family are maybe not just because of death, of noble sacrifice, but perhaps instead because those people who shared your blood became monsters, severed from their family because of their terrible actions, and still live as awful hateful versions of themselves, enslaved to evil, and that could be you.
And what do you do with that? Will you strike your father down with all of your hatred, when the thing that corrupted him by his hate for its own ends is sitting there grinning and laughing, waiting to do the same to you? Is violence the answer against that creature, infinitely better at taking advantage from violence than you are? Or will you just die - and even just walking away here means death, sooner or later - and let the evil persist?
Or will you, privileged young person with ideals and hopes, with a family member who has done terrible unforgivable things but who still holds affection for you, make use of that affection to tempt them to just turn their back on that evil for a moment, the thing it will never expect from the person it made its slave for longer than you've been alive? Neither you nor he can pay back the crimes of those years, but perhaps you can stop the evil, here and now, from going on.
So you do that. And what is your reward? Is it appropriate for Luke, whose whole story has been about becoming the ideal he grew up admiring and defeating the evil that ideal had the potential to become, both halves of it embodied in the being of his father, to come back to his friends and then have the universe say to him 'your father was unredeemable, and had nothing good enough in him to deserve peace in death'? Or to say there was a darkness lifted from him, and a light restored?
The whole purpose of Darth Vader in the story of the original trilogy is to represent who Luke could be, and through Luke, the audience. He wasn't really supposed to have a character arc of his own, his redemption isn't for his own sake, the story isn't about him - or wasn't meant to be originally, in any case. How you depict the fate of Darth Vader is something that sends a very strong message, and there's a reason why it was chosen as the final message of the original movies, in the context of the world in which those movies were made and who they were intended to be speaking to. If you change that, you change the message. Which you can do! And you can take issue with the original message! But like, there was a message, that was chosen purposefully, and you have to lose the original message to add a new one.
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