I think I finally found the problem and fixed it!

Since it didn't cost much, I replaced the derailleur hanger -- but the problem persisted. I checked the alignment of everything, but everything looked okay. So I started messing with the rear derailleur adjustment again (Shimano 6800 GS Di2). I've been using Di2 for five years and it's always worked perfectly if I adjusted it according to the book, but why not give it a try?

Since the chain was mis-tracking toward the inside, I decided first to adjust the derailleur closer to the bigger cog. (Adjust it toward the bike until it makes noise, then come out just TWO taps.) No help. Then, I decided to do what made no sense to me -- I adjusted the derailleur farther from the bigger cog. (SIX taps.) Voila! The noise went away. At least on the stand, no more mis-tracking. I rode the bike down the block and back, no more mis-tracking.

Tomorrow, I have a 90-minute hilly Tempo ride scheduled. That should give me plenty of opportunities to test the issue. If anything is left of the issue, I will try adjusting seven taps from the "noise point." I think I'm close to having this problem behind me.

What concerns me, though, is that I don't know the real cause of the problem. Maybe it's a function of the chainline geometry on Niner's RLT? Maybe of the geometry of a wider rear wheel spacing? I don't know. I just hope it's cured and doesn't come back.