1. A .50 cal coming in at the angle a plane uses to strafe is going to have a hard time penetrating a medium tank let alone something like a Tiger. A cannon on the other hand has a better chance, but even at 20mm its going to have a hard time penetrating at an angle.
2. Of course this could happen but you make it sound like it was a common occurrence, German rifle fire I am sure was especially ineffective, that's just silly.