Welcome aboard Revenant342!
We're always glad to have newbies to make harras, er, I mean, help.
Credit for a kill. I've been confused about this since I started playing. Basically, if you are in sensor range, you get credit for a kill. You may even get credit when the ship sinks out of sensor range. Storms and rough seas have an effect on getting credit. You may have crippled a ship, but a rough sea dealt the final blow. There was one time, I had crippled a ship. Ready to launch another torpedo, then I saw in the periscope, an allied plane fly over and drop a bomb, thus sinking the ship. I didn't get credit.
Tankers and military ships. You can get lucky with the military ships and hit ammunition storage. With tankers, if you get them full, one torpedo should get it with a real pretty fireball. If the tanker is empty, then it may take 4 or 5 torps to sink it. Remember, a tanker is designed to hold liquid and still float.
Detection range. Your assumption is correct, but don't rely on the information. It is only an estimate.
The last question applies to the U-boat campaign. I don't play it, so I can't answer your question.
Now as newbie, you can go polish the lenses on the periscopes (the outside lenses).