As to what U-Boats did in reality i'm no expert so i'm afraid i can't help you with your first question.
As for the thermal layer i've read a lot of post about it and it just isn't modelled in the game. I believe even a modder tried to look for it within the game files but didn't turn up anything.
I've played out one career until the end of the war and certainly from 1944 until the end it can be a real challenge to evade those DDs.
One way is to change course and increase speed directly after the first depthcharges have been dropped. As you are now behind the destroyer it has lost you for a while and in the noise of exploding depthcharges it gets more difficult for them to hear you. Once this is over return to silent running and hope for the best. Also, you have countermeasures available to you ( Bold ) which could fool them into thinking you are still in a previous position
During all this another point is to change depth every now and then. I've found that going deep and eventually coming up again can also loose them. But nearing the end of the war it can get a real challenge.
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Den Tipperary song, wenn ich bitten darf !
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