Hi Eric,
minimum distance for firing is a tricky business. Torpedoes are usually fitted with a device that prevents them from arming while still too close to the firing boat. Another factor to be considered is that there is an intial "dip" to about 20 metres following the firing, after which the torp will rise to the preset depth. In my experience as torpedo officer I'd you don't want to fire under 300 metres.
Average range of firing is an imponderable, you fire when the target is there since he certainly will almost never oblige you by coming to your choice of firing position.
As for max range, there are different combinations of speed settings. The G7e T1 could run 6000 m @ 44 kn, 8000 @ 40 or 14000 @ 30
The G7e T2 electric could run 5000 m @ 30 kn.
HTH, Brian