1) Yes, they are reliable. Internal hubs don't require much maintenance, are cleaner to operate, and don't have an external derailleur to get hung up on things when folded. The Sturmey Archers do have one adjustment that you have to maintain, especially when the cable is new. Simple stuff.

2) Yes, changing the rear is slightly more time-consuming. It's not hard, not rocket science. Just have to remove the wheel.

3) You can change to a short cage, as long as your rear cassette is compatible with it. You can go up to a 27t on the rear in the Shimano world.