I am only really familiar with how WavPack handles stereo, and how the “extra” modes work, so I’ll clarify that a little which will hopefully add something to the discussion.
In the “fast” and “normal” modes the default behavior (as was guessed) is just converting left-right to mid-side, and then treating the two channels completely independently. It can be turned off for comparison (-j0), but it’s almost always better. All of the “extra” modes check to make sure mid-side is improving things.
There is obviously still going to be some correlation between the channels even after mid-side encoding, and so the “high” and “extra high” modes take advantage of this. The filters with negative term values (-1, -2, and -3) employ this “cross-correlation”.
As for the “extra” modes, when I created the filters that are available at levels -x1 to -x3, there was very little high-resolution material out there (I think I had three tracks I captured somehow from a DVD) and so I didn’t use that in my corpus. Everyone was just comparing compression using CD audio and so I optimized for that.
The higher modes (-x4 to -x6) create new filters from scratch, so it makes perfect sense to me that those would be best for high-resolution (they have no preconceived notions).