As you said, there are people here who are well read in regard to U-boats and tactics and will most likely know the answers. I would think if a U-boat or WW2 submarine were under attack or in danger of being detected, they would engage in evasive action, initiate silent running and communicate quietly if needed, hand gestures included.
That's supposition on my part though. I'm guessing the questions are part of your efforts in making a U-boat management-style game as you mentioned in "WW2 Sub sim game vs. RL U-boat captain ".
http://www.subsim.com/radioroom/showthread.php?t=228700
If that's the case, best wishes to you and in it's development.