I use Premiere CS6. Most of my work requires simply editing with one audio track used for the on-camera talent's microphone, adding another track for music, and then outputting BOTH of those tracks to the right channel as a mixed mono output AND the same for the left channel (an identical mixed mono output).
Otherwise, I have had problems in the past based on a user's particular playback settings/hardware/etc., and they may only be able to hear one channel or the other (only hearing voice or music). If I can put a mono mix on the right and a mono mix on the left, then I am covered.
SO HERE'S THE QUESTION: What settings do I need to adjust to accomplish this, and how do I set this up as a default for future projects? (I realize that Premiere CS6 has a lot of flexibility when it comes to controlling audio, however, I have read several help articles, and cannot find the answer to this simple question.)
You're overthinking this. A normal stereo mix with the tracks centered will do the trick. There are several ways to center the audio. Option 3 is probably the most efficient.