Thanks, Eric! In the meantime, I'll see if I can pop open that handlebar without damaging something else. Maybe a look at the insides will show me exactly what's wrong, physically..
I mean, I'm pretty sure I can guess HOW it happened. (must have knocked the bike over at some point while it was in folded position, and the left handlebar impacted something) but maybe the damage itself will be an easy fix, or I can at least take it out of the "down" position.
Update: Huh, looks like there's a single simple microchip attached to the trigger that is separate from the upper sections of the handlebar controls. There's a small piece on the bottom of the microchip (between it and the outer trigger) that seems to be damaged. I could just unplug the entire trigger microchip section to disable that trigger. That should do the trick, for now!
Update 2: Unplugged the microchip and set it aside, successfully disabling the trigger input. River Run is now playable, sans my inability to dive.
I don't suppose you guys still have access to individual replacement parts for these things, do you? If I could get a replacement of that one tiny part, I think I could fully fix the trigger myself. I'd pay for the part + shipping, of course! :-D
Functionally speaking, making one of the left handlebar's face buttons an alternate "B" input would be an easier solution to the problem, but I'd get some personal satisfaction out of fixing the trigger, y'know?