It seems a little odd to me to present #3 and #7 at different levels, even though they’re the same. I would be more confused by that rather than less, if I knew even less than I do (which isn’t much).
I understand and no matter what you decide, I respect your decision. Still, while this may help users who don’t know the maths, it can be very confusing for those who do understand the maths.
I agree - minus 6db is a good idea… But the only reason for the extra option would be if someone wants to hear L-R without the 6db reduction. I don’t know how important that is to people, but if it is important, it could always be like this below.
This nice thing about this, is that people who DO know the maths will not be confused, and people who DO NOT know the maths will not be confused either.
Default Channels
Mix to Mono [(L+R)/2] Mid Channel Only
Mix to Mono [(L-R)/2] Side Channel Only
3a) Mix to Mono (L-R) Side Channel Only