I think this is a really cool “what-if” idea.
You will be the one putting all of the work into this and I think in the end you should do what makes you happy, but I also think it would be cool if you give yourself some guidelines to follow when making this edit. You are already doing it as far as trying to stick close to what Gary Kurtz described. Even though source material with his influence is limited since he left before pre-production began on “Revenge” of the Jedi, from my understanding, but you could also pull from the early drafts of the film as well. Descriptions can be found here.
http://www.starwarz.com/starkiller/draft-variations-for-return-of-the-jedi/
Based off what I’ve read over the years, I might have some useful suggestions.
Endor originally was a resort moon that revolved around the Imperial capital. A remnant of this remains in the final film, when the Emperor refers to Endor as “the sanctuary moon”.
So, the plotlines around Endor and Had Abbadon could be in relatively the same place. I can’t remember the original mission, but the rebel mission could have two pretty solid options. Either the rebels could either want to liberate the capital or kill the Emperor, or both!
Maybe the shield generator surrounds the capital world, and they want to take it out in order to have a ground invasion. (I know there is some concept art of some small orb stations around the planet that were meant to be mini death stars, but maybe they could be implemented as security stations or something. The generator could even be powering the security orbs, but that would be a lot more work. To keep simple, I would stick with a planet-wide shield. While you don’t have to show the ground invasion, you could use shots of Coruscant from the prequels or the Special Edition ending.
You could either have the Emperor’s throne room be underneath the City planet like it was depicted in early concept art (which would be A LOT more work), or it could be on the SSD like others have suggested. While Luke could still be viewing the destruction of the fleet on a viewscreen underneath the planet, I think it would work best on the SSD, and it makes sense because the space battle would be taking place in between the city planet and the moon.
And like others have suggested, you could use the briefing scene to get this exposition across to the audience. This scene is probably one of the most crucial pieces to your edit. While dubbing General Madine is an option (but you might worry about having the lines match the mouth and having enough footage) you could possibly create some background plates of the briefing room shots and green screen in a new character to deliver whatever information you want. Maybe get rid of Mon Mothma and Madine if necessary. If you don’t think that’s possible, it might be more believable to redub Admiral Ackbar than Madine and have him deliver that info too. I would even have in the opening crawl that despite their losses at Hoth, the Rebellion has grown in the interim to make their assault on the capital more believable, and possibly add more ships to their fleet.
Another problem with these types of edits though is that the run times become incredibly short compared to the original version, so I would try to find a use for as much footage as possible. Maybe Lando could fly into the SSD to destroy it instead of the Death Star, for example. This way you could also use the Vader death scene in the hangar as everything is falling apart around them.
While you were thinking of having the Rebels be defeated, I would honestly have them win. This is the “final” film in one of greatest film franchises of all time, so I think the stakes should be at their highest and the ending be satisfying. Having the Rebels win, but Han die, Leia have her greatest loss at her greatest victory and Luke go off on his own is what I think Kurtz meant by bittersweet. Victory at a cost. Maybe you could get a shot of the Imperial Fleet scattering into hyperspace after the SSD is destroyed, implying the fight might not be over though.
And far as the ending goes, having the two plots physically close together would also make Luke burning Vader’s body on Endor make more sense. As far as Leia, maybe instead of making her Queen of “ashes”, she could be declared the leader of the New Republic on the capital city. Even though it isn’t exactly what Kurtz described, it definitely would be a natural progression of that idea, and fit in more nicely with the narrative. You could possibly rotoscope a quick shot of her in the senate chambers or in a more OT-inspired environment. Maybe she looks serious, sad even, because she has lost the love of her life but has this responsibility she has to deal with now.
I think you should kill off Han, because that’s what Kurtz wanted, but it would be cool if you made an alternate ending where he lives still.
Also, I like your idea about making Vader more against the Emperor, but how would you justify him protecting the Emperor in the throne room? I always assumed that maybe the Emperor has some kind of influence over the Imperial Fleet to keep them organized, and that killing him during the battle could send the fleet into disarray and give the Rebels the upper hand. Maybe that concept could be implemented somehow? Some new lines from Vader on Endor?
I also like the idea of not making Leia Luke’s sister. I feel like Kurtz didn’t like this either. The easiest way to do this is to cut any mention of Leia from Luke and Ben’s conversation, and when Luke reveals it to Leia. You honestly wouldn’t have to change anything else, and the ending could imply that Luke is going to go find his long lost sister and restart the Jedi with her. Pass on what he has learned. An Episode 7 for an alternate reality.
I’m so sorry for the long post, but I think this is a really great idea Snooker, and I just wanted to share my thoughts on it. I think the three big things to stick by are:
To use Kurtz’ interviews, early drafts and concept art as primary sources of inspiration.
Use as much of the original footage as possible to maintain a believable runtime.
The simplest solutions are probably the best solutions.
I think what could make this edit stand out is by sticking to a set of rules like these, because it’ll make it feel like a true “Revenge of the Jedi” almost from an alternate reality. But it’s your edit so you should do what you’ll enjoy the most. It’s a cool idea and I look forward to what all you end up doing!