I don't get it, you say you know people who've served on U-boats and yet you insist on the possible existance of a German DD in the middle of the Atlantic?
"#2) What is it doing there? Any number of possibilities. It could have been part of a convoy of some type as anti sub protection.."
Give me an example of one German convoy in the Atlantic during WW2. There are none. And certainly not at this point in the war. There are some things that could be called 'convoys' maybe close inshore, for example all u-boats that left port would be preceded by a big ship full of empty barrels to clear mines. It is possible that a destroyer might come along too, for sport. But that would end before being out of sight of land.
"it could have been the escourt to a ship like The Wilhelm Gustloff--even though that would be farther out than would be believable for the Gustloff to have been sailing."
And what, pray, would the Gustloff be doing in the middle of the Atlantic (by then a hospital ship, BTW). There were no German merchantmen in the Atlantic. They were all interned or sunk or had managed to get back home in '39. A few - very few - resupply vessels managed to stay around until 1941, I believe.
"The Zerstorer could have been on a rendezvoux of somekind...maybe with a wolfpack?"
And what could the DD give the wolfpack? The DD wouldn't even have the range to reach a wolfpack that is in the middle of the Atlantic. And rendezvous between DDs and SSs never happened, AFAIK.
"there were still wolfpacks at that stage of the war. It could have been somekind of a picket ship on patrol...."
Come on, patrolling against or for what? If you're thinking of having a DD in a sort of u-boat supportive role (I'll find the convoy, you go get it) you must realise that the life expectancy of the destroyer is pretty close to zero as soon as it has found one.
"or, it could have been on its way home from some other place?"
There were no German DDs operating abroad in any place that would force them to cross or sail through the middle of the Atlantic.
"or it could have been there by chance??"
I'm afraid that this is what the makers of the film decided should be the case.
It's sort of sad when you think about it!