3Jane makes a good point.
Firing from close distance has two pro's: you are far more likely to hit the target where you want it to be hit, plus the chance that a torpedoes goes boom before reaching the target is very small.
But when you are targeting a convoy with escorts swirling around, it is another story. You definitely want the longest time between the moment you dive and the moment the torpedoes hit the target, because you want to be as deep as possible from that moment.
Personally I fire at single merchants between 500 and 1000 meters.
In convoys, That's usually 1000-2500.
The other question...I haven't tried that out, but I heard ppl say yes and others say no.
