The Eclipse idea would actually be the easiest idea to implement. It would require the least amount of changes, and the briefing room dialogue would only need to be slightly altered (still would need to change the holograms though).
While the look of the Eclipse wouldn’t necessarily have to be how it looked in the EU, it actually could have a smaller Death Star laser like it did in the EU. Then Lando’s surprise at it firing would make even more sense. This kind of surprise would be easier to get across than the cloaking device, I think, and give more purpose to the Endor base by still having it be a shield generator. And it also feels like a natural progression of Imperial technology, with it being a fusion of the Death Star from Star Wars and the Super Star Destroyer from Empire Strikes Back.
I imagine it could’ve been a compromise of ideas between Lucas and Kurtz in another timeline.
And if it was under construction, you could still have Lando fly inside and blow up its main reactor. This version would allow you to maintain the max amount of footage, really.
I think taking inspiration from the Kuat Drive Yards would be pretty cool, but it would make for a bit of work. I would suggest going even simpler. One thing I would consider looking at is the design of the Fondor Shipyards from the 2017 Battlefront 2.
To me, though, the easiest thing would be to just have the Eclipse, but then have dozens of small specks around it that are meant to be parts and construction ships. Things look busy, underway. I think the only downside would be that you would have to invent a new way for the Executor to be destroyed. With some kind of space station, you could just have it crash into it like it did in the movie, but you might want to have it blow up differently anyway since people felt like it’s original destruction wasn’t satisfying.
And by not having them on the Eclipse instead of the Executor, you wouldn’t have to remove it from Luke’s view of the battle through the window. A small thing, but one less thing for you to do!
But by just having the Eclipse, all you would need is one good model (real or CG). It might be easier to find models of the Executor, and touch it up some to differentiate it. The level of detail might be important, because I would worry a CG model would stand out amongst all of the traditional model work.
And thinking more about it, I totally understand more why you don’t want the Rebels to outright “win”. I also went back and looked at more interviews with Kurtz, and he at one point described an ending with the “rebellion in tatters”. So maybe they accomplish their goal, but at a heavy cost.
One idea to emphasize this would be to have the Rebels leaders get killed. Madine, Mothma, maybe even Ackbar. It would only require you to insert shots of them before we see a ship blow up. Seeing these important people be killed would definitely get that point across. The death of the senior rebel leaders could also be good way to have Leia be left as the leader of the Rebellion. I know Kurtz said she would go off to become Queen of what’s left of her people, but not only would that be harder to get across to the audience, but at this point I think the rebels ARE her people, and despite all she has lost, she still wouldn’t abandon the cause if the war was not over. So I think this would be the most natural ending for her. Maybe you could have a shot of her, standing alone on the bridge of one of rebel ships.
Once they destroy the Eclipse, Leia and the surviving Rebels meet up with the few remaining ships of the fleet and jump to hyperspace. Luke, on the other hand, stays behind and lands back on Endor to have a funeral for his father.
All of the stuff with Hab Abbadon could just be abandoned. Even with Kurtz, maybe this concept still would’ve been left behind. In retrospect that would be WAY too much work and hard for it to look believable.
Sorry for another long post. It’s fun discussing this and brainstorming with you guys. Hope you guys are enjoying it as much as I am!
JEDIT: Another, simpler, option for the Executor could be to just modify the existing shots of the Executor with photoshop to differentiate it more.