I like that “two movies” idea a lot! I think I’d go about it a little differently, namely to allow Vader’s redemption a bit more development and show him trying to fess up to his mistakes. I like your ideas, Outbound Flight, so allow me to build off of them.

Episode III: Labyrinth of Evil

Do the whole rescue of Han deal, working into Han and Lando trying to mend their relationship. It could be a good piece on how Han’s character has grown and the desire to take action rather than sit the Galactic Civil War out like Lando was content to do. You even have them tag-teaming Boba Fett to end their arc, with Fett dying pretty conclusively (e.g., not much left of his body).
Have Vader fall out of favor with the Emperor (over space skype), as Palpatine replaces him with a new apprentice (call him Maul, because I’m not keen on prince pheromone from SOTE). Vader is a fugitive, with nowhere left to go in the Empire.
Maul tracks the heroes down to Tatooine, with Vader in close pursuit. Vader uses this time to reflect on how Palpatine has thrown him away and how he must join with his son to defeat the Emperor. We get flashbacks to his time as a young man, before Palpatine took him under his wing (different from the Prequel setup, obviously). These are the first inklings that Vader might be regretting his choices.
Vader finds Luke and Leia, telling them what has happened. He announces his intention to betray the Emperor by aiding the rebellion, which both Luke and Leia have some doubts about. While Luke can sense that Vader is serious, he sees that he’s doing it out of selfish reasons. Throughout the movie, as he feeds correct information to the rebels and helps out Luke and Leia, we see that he can be trustworthy yet still within the dark side.
Leia sees how pathetic Vader is and how he had little autonomy under the Emperor. He couldn’t fight the order to destroy Alderaan, even if he tried. She sees him as a useful resource but believes that he should be condemned for his crimes against the galaxy.
Pepper in some flashbacks about Vader’s past, with Luke learning his name from locals and confronting Vader with the revelation. He says the whole “That name no longer has any meaning to me” bit from ROTJ, but Luke doesn’t do the whole “true self” deal, not yet.
The movie would end with the defeat of Jabba and Maul confronting Luke and Leia. Luke would be outclassed in the fight, with Vader intervening to kill Maul. It would confuse Luke, as he did not feel Vader acting through the dark side. We also might see Leia using the Force, inadvertently, for the first time since the ESB telepathy.
Our last scene would be Luke and Vader at the ruins of the Lars Homestead. This is where Luke tells him about the good he sensed in him, begging his father to “come with him” and tease it out. Vader merely responds with “It is too late for me, son.” Luke would respond, “But is it?” The heroes travel back to the rebellion, while Vader remains and meditates on this discovery.

Episode IV - Return of the Jedi

With Vader continuing to provide the rebellion with information from a distance, the Empire (now revealed as the “Sith Empire,” which invaded and conquered the Republic - borrowing another Outbound Flight idea) has been pushed to a few straggling planets, one of which is the Wookie homeworld of Kashyyyk. We see this through a montage of imperial worlds falling to the rebels and the destruction of Star Destroyers/imperial artifacts. Learning that the Emperor is aboard his skyhook orbiting Kashyyk, the rebels plan to attack.
Han and Leia lead the strike team to take down the shield generator and allow the rebel fleet to engage the last of the imperials (think Jakku, but it’s Kashyyyk instead). Han and Leia’s arc revolves around Chewbacca, with Han believing him capable of convincing the Wookies and Leia doubting that an exiled Wookie will win them much support (Chewbacca led a failed rebellion that resulted in the enslavement of many of his kind, including himself).
Yoda and Luke meet, with Yoda dying after telling Luke about the “other Skywalker.” Obi-Wan then confirms it to be Leia, while also telling Luke that turning Vader is impossible. Luke then joins the others with the rebel fleet, telling Leia about her origins and how she can use the Force to further the rebellion’s goals.
Luke then tries to find Vader to help him face Palpatine. He finds Vader on Mustafar, the planet where he sustained the injuries that forced him into the life support suit. This is an opportunity for both a flashback (perhaps to his fight with Obi-Wan) and for Luke to remind him about the good he sensed in him. Vader tries to push that down, encouraging Luke to join him in the dark side. They resolve to confront Palpatine and overthrow him, albeit for different reasons.
Perhaps Vader converses with Flashback!Anakin, in an inversion of the Dark Side Cave from ESB. He sees a vision of what he could be had he not turned, looking a lot like David Prowse.
The Palpatine-Luke-Vader scenes play much as they do in our ROTJ, albeit with Luke and Vader coming to blows because Vader fears Luke will be too powerful for him. They fight, with Luke almost giving in to the Dark Side after Vader threatens to turn Leia. The rest of the scene plays exactly as it did before, with Vader realizing the good within him and tossing the Emperor into the skyhook’s reactor core.
The ground battle plays much as did in our ROTJ, albeit with less cutesiness and more strategy from the Wookies. The shield comes down, with Leia coming around to Chewbacca’s plan and using the Force to help access and destroy the generator. No main characters die here.
The rebel fleet attacks the skyhook and the fleet, as Lando leads them to destroy its core. The Executor also severs the skyhook from the planet, as a turned Vader uses the Force to influence Piett to ram the ship.
Our ending is the same, albeit with no jealous Han and Ewok cutesiness. The spirit of Anakin (Prowse) appears to Luke, with Leia catching a glimpse of him as well. Music would be a triumphant version of “Luke and Leia.” The camera pans up and into the stars, starting the credits.

TL;DR: Draw out Vader’s turn, have him act in ways that help undo the Empire’s damage, and give him more moments of reflection. A bit Zuko-ish, but it works. Tangentially, give Han more of an arc with Lando and Chewbacca, while also letting Leia use the Force more.