I have been playing SH3 with all the GWX incarnations over the past several years. I have noticed at times aggressive DDs regardless of the year. I always thought it was just the way the game was. Then at the recent Subsim meet in Texas a friend of mine, also frustrated by being pinged for no apparent reason asked a simple question....
"How can they possibly be pinging me?? Why ??"
The answer was even simpler....
"Because that is what destroyers do."
So a DD that even
thinks theres an enemy near, is going to ping like crazy regardless of your position in the hopes of catching you. Just because you are under the cone doesnt mean the DD will stop pinging. Think if you were a DD Captain, being ping happy could save lives.
As far as your death by Depth Charge problem, maybe the previous detonations you heard where above your depth and the last one just floated down and nestled next to the control room before it exploded. I one hit Depth Charge kill is certainly possible.
Everything you have described seems pretty real to me, most of all, your frustration. If I was a U-boat Kapt. I would be bald from pulling my hair out in frustration.
Of course thats just my opinion. I have never played missions or multiplayer. I am strictly a campaign person. All my above info is derived from Campaign play. I can only suggest playing that same mission a few times and see if the AI behaviour persists.