How do you guys react if you hear/feel something suspicious happening down at the crank or the rear when you're really moving it, particularly if you can't really afford to take attention away from the road? Hit the brakes and hope for the best? Brace for impact? Pretend it's the wind?

Do not panic. Even at speed you should be comfortable enough to be able to look down (or not look) and grab your water bottle.

On the other hand, if your carbon fork snaps, you won't have time to react. You'll most likely wake up in the hospital and will have no recollection of what happened.

hammer it for a few seconds then jump off the bike doing 40 = win. do that.