Sluggo, thanks for the 8mm recording, I listened to it this evening. It was pretty bizarre hearing the film cut down into what essentially amounted to a "highlights" reel, lol.
Having now heard the soundtrack, I can weigh in with my opinion. While the differences were very interesting, I still find myself disagreeing with the notion that they accurately represent the 70mm version. They probably did derive the image from the 70mm sources, going by the pics of the Emperor being already tuned in and the missing radar dish, but I can tell you right now that the audio track on this 8mm recording is definitely its own separate mix. It is not simply transferred from the 70mm soundtrack!
How do I know this? Several reasons--the first of which is that the music is clearly mixed separately from the sound effects and dialogue. There are several instances in which the music is edited and synchronised in ways quite different from how it appears in the film itself. These edits were made specifically to accommodate the shortening of the film from its full length--they are not simply jump cuts from one shot to another, although there are numerous instances of that also. Second, the Film Score Monthly list specifically notes the absence of tracked music from the cue "Yoda and the Force" in the 70mm version for the rebel fleet at the end . . . and the 8mm soundtrack quite obviously does contain this tracked music. Moreover it accommodates the tracked music by shortening the cue Williams actually intended for the scene.
I haven't seen the image to go along with it, but I had a feeling as I listened that some of the alternate dialogue, such as C-3PO saying, "It's so nice to be together again," could only be present due to editorial differences in the picture. There's no place for it in the full movie, and no mention of that scene being edited differently in the 70, either. Interestingly, I did recognise some extra Chewbacca vocalisations that were in the SE version, and not in the 35. It's certainly possible that some of the alternate lines, in the 8mm and/or the SE, originated in the 70mm mix; but given the obvious ways in which the 8mm is uniquely different from the others, there's no reliable way to tell which lines actually came from the 70 and which did not. I certainly would not feel comfortable or justified inserting alternate dialogue into the film and calling it authentic--and I would say that even if the fidelity were good enough that such insertion could be done seamlessly. As it is, the sound quality does not permit it to be done without calling attention to itself.
It looks as though reconstructing the ESB 70mm mix accurately is not yet possible, unfortunately. If it were I'd be glad to work on it, but I don't think it would be worthwhile at this point. That doesn't mean it isn't a worthy goal for the future, though--we just need more sources.