Called UA and was turned down to reroute IAD-ORD-ARN despite having only 9 minutes to transfer with a gate-checked stroller. Also would not allow me to drop the IAD-CHO leg (and drive to CHO) since this is how I booked the trip. In the words of the agent, "no sympathy". Not sure if it's because it's a more friendly airline or having elite status helps, but it's quite a contrast with my experiences with Delta. Night and day actually.
It looks like I could take the same plane back to IAD if we miss the ORD flight and still make our TATL (IAD-CHO-IAD-ORD-ARN), but that would be unpleasant with a squirmy baby and would rely on having open seats on the additional flights. But at least it's possible.