What is the difference between the 5.1 1980 mix and the 2.0 1980 mix? Are there any content differences, or are they just mixed through more channels?
Also, does the 16mm mono track differ from those two in any way?
IIRC, the 5.1 1980 mix is just the matrixed stereo upmixed to 5.1, with some of the LFE channel from other sources like the Blu-rays where they still seem decent. In other words, it’s a nice, tasteful upmix of the stereo mix. Unlike Star Wars, it is NOT an attempt to reproduce the original six-channel mix. And unlike Jedi, we know that there are content differences from the six-channel mix, but the six-channel mix didn’t apply to this cut of the film. So no content differences, just more channels, technically not theatrical but not actively revisionist in any way.
The 16mm track is way different. C-3PO gets some different lines (ones that later showed up in the Special Editions), and the mixing is occasionally a bit different too.