Buchheim had known Lehmann-Willenbrock for years before he wrote Das Boot, so presumably his depiction of L-W's character is accurate, at least from his point of view. As to the physical discrepancies between Lehmann-Willenbrock and Jurgen Prochnow, there's a picture of the former in Buchheim's U-Boat War/U-Boot Krieg where he does look a little like Prochnow. It's toward the end of the book and shows Der Alte writing the war log.

As to a composite U-boat commander character, lots of luck finding a Hollywood leading man that could match Schepke for looks. That boy made jug ears look like a fashion statement. :-)

Pat W