Your questions got me currious so I did me a litte internet search and found an article that may well awnser your questions, as well as correct me about that depth keeping device. As it turns out they used a mechanical pendulum and a hydrostatic valve in combination to set and keep the depth, though they didn't go into great deatail on how those things work, sadly as that seams rather intresting.
Here is the article if you are intrested in a bit of reading.
http://www.uboataces.com/weapon-torpedo.shtml
I have to go to a class just now so I didn't get a chance to read all of it but there is a section about giadance and some pictures that may get you the info you want about how they programed the torpidos. It seems they had to program them befor they put them into the tube, thoguh I can only base that on a picture I found in that article.