I've been playing as german also, this is what I've seen:
1) I've been doing this by simply moving the plane to the area you want covered and leaving it there. it can't be a place with any enemy units in it though, because then it would lead to combat and the plane would go back after the combat. just leaving the plane over water at the end of your turn is all you have to do. I never tried it, but I presume you can leave the plane over enemy held territory as long as there is no combat (i.e. enemy units)
2) op-fire happens automatically. it happens at more than just narrows, island will do it also.
3) subs seem like germany's best weapon. they just play like any other unit. I haven't been moving them around a lot, like you have after attacking though. its seems best to use them in large groups though because each subsequant attack on an enemy drops the enemies evasions. therefore with all the subs in the beginning having a weak attack, the first sub attacking is going to miss for sure. the second might hit, but the third (and others after that) will surely hit.
My first game, I went wild on researching my subs up. I started to feel the game was unbalanced for a time because anything my sub packs came across was blown out of the water as destroyed (not even damaged). it wasn't until 1945 that the WA started getting antisubmarine warfare really into play and then my subs were slowly wittled down. Of course by that time russia had steamrolled over me.
4) you can uses planes for everything. it really all depends on the numbers though. which is best at antiship, use for antiship. which is best for antiland, use for antiland. Unfortunately I don't recall which has the best values, but I can say that based upon range, its easiest to use the strategic bombers for antiship, since the fighter only have a range of one.