Was not the vital challenge of the night bombing campaign that of navigating to the target and then, having found it, putting the bombs on it? Unlike the US daylight raids, when you stayed in visual formation, followed your leader, and then "toggled" off your bombs in reaction to his drop, each night bomber was its own bomber leader, often unable to see other bombers though painfully aware of their close proximity.
It seems likely that is why Navigators and Bomb Aimers were invented. The Nav could concentrate solely on getting his aircraft to the target and then back to base. The Bomb Aimer could help him in that task, passing fixes, drifts, etc en-route, until able to perform his prime function of putting the bombs onto the target (the whole point of the entire mission). Manning a gun turret, his own or anyone else's, would have been a low priority in comparison. Certainly head on night attacks, to be dealt with by the forward turret, would have been very rare I suspect.
Of course electronic aids such as H2S, Oboe, Gee, etc, aided navigation as the campaign progressed, but counter measures and inexperienced crews still meant that simply finding the target and accurately bombing it was a challenge to the very end.