No. You are trying to explain a mechanical failure of a door right at the moment when the aircraft suddenly starts descending into mountains all the while during which the copilot also does nothing to try to correct this unscheduled descent and also ignores air traffic control. Seriously if it has wings and floats on the water and looks like a duck and quacks like a duck, it probably is a duck. Your version requires many, many things to go wrong at once. The simple answer is, of course, only one thing went wrong - the co-pilot locked the door and set the plane to descend. Occam's razor, and all that.
Adding fuel to this theory is that the co-pilot was detatched and monosyllabic when receiving the briefing about landing in Dusseldorf - he had already made up his mind that he wasn't going to reach Dusseldorf. If the pilot wasn't going to go to the bathroom he probably was planning on killing the pilot anyway.