So actually, I've done some homework.
Seems like the major peaky flows are:
-- Caribbean: 8-11am departures, 3-8pm arrivals
-- Europe: 6-8pm + 8pm-midnight 2 batches of departures, 11am-3pm arrivals
-- Asia: 11-1pm departures, 1-6pm arrivals
Other US destinations are relatively smoother over the day.
See below for the count of UA (mainline) flight numbers on a recent representative Thursday:
I would still like to know about the B vs. C issue though!