DT are watching this thread
In the case of the MK21 few RAAF Beaufighters ever carried a gun in the observers cockpit. There are just 2 images (that I am aware of) of MK21's ever the showing a rearward facing .303 MG but thats about it, one of the images (A8-99 over Princess Bridge Melbourne) is a little sus and looks as if it has been retouched. The other one shows a MK21 (A8-95) over the Melbourne Shrine of Remembrance that has a gun in the back but thats about it. Given only the MK21 is modelled in IL2 (and only the RAAF operated the MK21) then discussion of the early marks in RAAF/RAF service is perhaps redundant, however in most cases they also did not carry any rearward facing armament.
In RAF service the TF.X a rearward facing .303 calibre weapon was common, where a specific non bubble observers cockpit was fitted
The idea of Nav providing SA by Bandit sightings etc has great merit.