a) Comparable in both options. (about 20 minutes or so either case).
b) Same. Wifi costs in both places. IAD has a variety of hotspots, ORD is just boingo. ORD as a building is better, so it may be nicer in terms of looks/waiting areas.
d) If you're traveling in the winter, IAD is a safer bet. Otherwise, doesn't matter much. In either case, leave adequate time so you can have a small connection on the first leg and still make the second hop.