My guess is the Hancock was already in Europe and had participated in the summer offensive. It would have been needed to escort the ships carrying the US marines.
I'm not sure off the top of my head about a one way trip. All I can recall is that burning crude oil causes "issues" with the diesel engines. If they were able to reliably cross the Atlantic westward, my guess is they may have been able to return again too.
Now I think about it, the use of German ships with German crews makes more sense of the fuel issue. Logistical control stays fully in the hands of the Germans thereby guarenteeing they get the promised equipment, and keep the ships. Not sure if the value of those ships outweighs the value of the oil they burn coming back though...
