The barbette did rotate 360 degrees. The gunner in front, sitting behind the pilot, had a remote turret gunsight. And the radio operator, in the rear of the center nacell facing aft, also had a remote turret gunsight. One guy aimed at stuff in the front, the other in the back.
Check out that link I posted in the beginning, it shows the coverage arc of the barbette.
EDIT: And it wasn't so much that it could barrel roll with an engine out, but rather it could roll INTO a dead engine. In most multi-engine craft, that would be comitting suicide.