Access to the only fully preserved U-Boat in Kiel, Germany was not granted to the production. A scale reproduction of a WW2-era Submarine was built in Lake City, Seattle, WA.
No, but a good number are stand ups who work in the Seattle area: Robert Pidde, Claire Webber, Thomas Nichols, John Gardner, Chris Bender, and Chelsea Tolle. They were cast from Laffs and Comedy Underground in Seattle, WA.
Yes, the German Kriegsmarine flag hanging on the wall was captured from a German vessel during the end of WW2. It was loaned by a member of the cast, who's grandfather fought against Germany.
The song was written by Patrick Penta and performed by Patrick Penta on guitar and Martyn G. Krouse. Marty is singing along to a prerecorded track. The song was recorded a day earlier on the set of the movie by Umberto Lenzi, the sound recordist.