Ok, so it looks like I indeed have three solo-safe tracks receiving input via Bus 2 (the problematic summing stack). "Selection-based Processing A", "Selection-based Processing B" and "Prelisten". I'm unclear why these would be receiving input from Bus 2 since I didn't knowingly assign them. Perhaps I did it unintentionally when I was setting up the project?
Should I reassign their inputs or ⌃+click their solo buttons as you mentioned?
Also, any insight into what these 3 tracks accomplish would be greatly appreciated