1) ??? No idea.
2) You can shoot steam torpedos (visible bubble trail) at different speeds. The faster the shorter their range, but the less the targets time to take evasive manouvers.
U-boats used only slow and medium speed settings.
Electric eels have only one speed (28 knots ingame) (sic!), but they stay stealthy.
3) The U-Boot is in port for several days/weeks for resupply, maintenance and repair. Time to push it around and even take it out of the water.
http://historyarticles.com/gray-wolves-den/