Well...the main rule book is pretty clear. Jay you are correct. If you use an AA gun to shoot at aircraft you cannot shoot next turn. And according to the new FAQ. One cannot do other things too.
Also, regarding the top hatch open. The RB states pg. 64 "Tank Machine Guns" states in part. That the AA MG can be used as a MG fired by the crew from the hatches and over the side of the veh. This makes them open topped. Therefore, it seems to me that if using the AA to shoot aircraft does not make them open topped.
What do you think?