welcome dive,
to answer some of your questions i ship can sink if you are within i think 25km but you have to have ship sunk message to get credit.
Don't trust the mag torps till late 1940 but they are very affective. Depending on where you hit the ship it should go down. If it dosent sink on one i personally aim at a spot a little bit to the side. (more holes=more water in different places.) And yes it does depend on ship type. Aiming just fore of the bow mast usally ends up as a k.o.
How many fish. Well for small merchants usally one (or deck gun) medium to large 1-2 sometimes 3 and 1 if your lucky. Also if you cripple a shipin a convoy and are forced under it might sink eventually.
I pesonally go for ships on the way to my grid but not to far out of the way (feul) but yes that is how your supposed to get ships. I've found that theres nothing in my grid most the time.
Dont click on "return to base" as long as your within 25km. If your in range it should say "Dock at...." However if you do click on it dont worry. It wont say dock at and you should click on cancle. About the fuel, yes save feul for return and mabye extra just in case.
Hope this helps
. Welcome aboard --mowgli