somenewguy: If you examine the real thing, it would be very unlikely for any gunner, on the engine deck and behind the loader's hatch to awkwardly fire the MG mounted through the gun shield -- because there is nothing to keep it upright. If the loader's hatch wasn't propping it up, it could theoretically still have the MG mounted onto it -- but then the gunner (on the engine deck) would have to not only hold up the MG34 but also the gun shield.
Just consider the gun shield to be completely loose on its hinges and you'll see why mounting the MG through it, with the hatches closed -- is a non-starter.
And yes, assume the shield to lie flat on the roof whenever the loader's hatch is closed.