I'm quite certain that the stereo mix was done first. My reason for saying this is because there a few sounds from the 70mm version that do not appear in the 35. Most notably, during the first trench run, when Vader destroys the first of the Y-wings. The explosion is in two parts: first, the fighter is hit, the picture cuts to the pilot reacting to his imminent destruction, then it explodes. In the stereo mix (and the similar '85), there is a second of near silence during the pilot's reaction; in other mixes, this shot is accompanied by a loud screeching sound. I first heard that effect in 1997 and thought it was an SE addition, and then later thought that it was one of the '93 alterations. Imagine my surprise when I heard it on the in-theatre 70mm recording!
Since it makes little sense to suppose that this sound would have been removed from the first mix to the second, only to later reappear in the mono version, it is logical to conclude that the six-track was not the first to be completed. The differences between them are nowhere near as pronounced as what was done to the mono mix, so it's not difficult to believe that there wasn't any more time to make changes before it had to go out. I can't say I'm all that fond of the mono mix, partially because many of its changes sound like "special edition fixes" to me, though some of them do add positively to the film.
On to progress: as I thought, the quad laser scene was somewhat difficult to get right, but eventually I managed it. My first thought was that I would have to use the 35mm for most of the scene, but I refrained from this as I would have found the dynamic loss unacceptable. Instead I listened closely for ideal switching points in order to eliminate the additions while keeping the rest of the '93 version intact. I've uploaded another test file for this scene (in 2.1 format again), because I'm curious to know how successful my efforts were. I can't say it's 100% seamless because of the difference in music reverb between the two sources, but it works much better than I originally expected. It's probably about as good as I can make it, but I want to know if that is good enough.
Download here: http://www.sendspace.com/file/y6yrtg
Only a few more changes to be edited out, and then the LFE channel, and this thing will be done!