The problem hinges really on how central the Death Star 2 is to the plot of ROTJ (annoyingly repetitive as it may be), as the macguffin being protected by the shield generator in the first place and the cornerstone of the emperor’s trap for the rebellion (he doesn’t just want to lure them into a space battle, but specifically in to one with (what they don’t yet realise is) a ‘fully operational’ and pretty much impregnable battle station. this specifically translates into the tactics of the endor battle; the plot necessity of lowering the death star shield and the need to engage the imperial starfleet at close range rather than being picked off by the super-laser. So, even in the spirit of radical redux, some kind of macguffin in place of the DS2 really needs to be kept for the plot to revolve around (maybe not another death star but some kind of powerful super-object/weapon; I know there was some talk in the sequel thread about making starkiller base a ‘star forge’ so maybe it could be some kind of reactor - Coruscant’s main energy source or something which doubles as a big gun in palp’s evil plan?). So the third act structure in this proposed outline would have three main locations; the Capital with Luke, Vader and the Emperor; the ‘Sanctuary moon’ with Han, Leia, chewie and the droids; and the ‘DS2’ (or equivalent) with Lando et al in orbit of Coruscant/the sanctuary moon.
You can see here why the choice was probably made to condense the luke/vader/emperor action into taking place on the DS2 - as it means one less location to establish. But in the spirit of radical redux I think it could probably be separated out again in a more ‘workable’ kind of way; set all the vader/emperor stuff in ‘the capital’ with new establishing shots, maybe tweak some of the mon mothma briefing exposition to set up the relationship between the capital planet, the moon and the macguffin station (making it a power plant rather than a new death star for instance if you wanted to go down that route). Luke and co would then go to the forest moon as usual, the emperor would instruct vader to go there to get luke and luke would return with him to the capital. as an aside; It would be fun to see the emperor’s throne room as the spire of the imperial palace (the former jedi temple) and provide a nice mirror to anakin skywalker’s fall in ROTS by setting his redemption in the same place (as well as seeing what’s happened to the jedi temple under 20 years of imperial rule). In such a way you probably wouldn’t have to change much of the throne room scenes (you could just say that the windows are dark because its nighttime, maybe add a few building lights in the background here and there); though the underground lava chamber idea is also cool if more difficult to execute. The battle then continues, perhaps with Han and Leia sacrificing themselves to bring down the ‘DS2’ shield, lando et al destroying the reactor and proceeding to storm the capital as vader kills the emperor etc…