[*]we also have an earlier than i would like dinner reservation one night and will have to get from either mk or cbr to downtown disney - suggestions on travel?
when going to dtd from mk, i usually take the bus to saratoga springs and either walk (about 5 mnutes' worth of walking from the congress park bus stop) or take the boat (as preferred by both my mother and my nephews, mostly for entertainment value). Total travel time either way is about 40 minutes from mk.
[*]how does one go about calling a cab - is there just one company that services disney, or can we call any local cab?
all resorts perpetually have cabs waiting near the lobby entrance. That being said, this doesn't really help at cbr since you'd have to travel over to the customs house to get to them. If i need one, i usually call for one and tell them to meet me near the bus stop for whatever village i am in (so as to have a highly-visible meeting point).
[*]will we need to request a car seat in a cab or do most disney cabs just have them available by default?
in my experience over the years, none of them have a car seat. Kind of like the me buses and the wdw buses, they don't expect you to use one. If you want to, you'll need to bring your own.