As for torps hitting the same spot, yes the 2nd and so on do some additional damage, so much so that your fourth eel caused enough for the section to be destroyed and the ship to go kerplooie...
Generally it's wise to use two eels (but not four, that's overkill) for any merchant over 4000 tons or so, and to try to spread you hits a little.
I usually aim one at the smokestack (to knock out the engine if it doesn't sink right away) and one at the forward mast (this usually hits the forward hold, which then floods, sinking the ship)
Every ship is different though, and you'll have to find out what works best for you, magnetic or impact, one or two eels, and where to aim them.
One tip, some of the shallow-draught ships (I'm looking at you small tanker!
) have a very rounded hull which makes it easy for your impact pistol eel to bounce off and misfire, these should probably be attacked with magnetic pistols....