"If a tail gunner could not see a German night fighter approaching in the dark I fail to see how a ventral gunner would"
A tail gunner could, when in luck, pick up an attack from astern but not one were an aircraft made a climbing attack from below. As the target was the wing fuel tanks, an aircraft was more likely to be lost. However, even after aircraft returned with battle damage that after tests confirmed cannon fire from below, it was not at first believed then seems to have been ignored.
Fitting radar to assist the tail gunner see in the dark had the opposite effect by illuminating the aircraft and with it the bomber stream.
Bomber Command was still being hit hard by the Luftwaffe well into 1945 although NF Mossies helped reduce losses. Electronics were seen as the answer perhaps as it was easier to fit black boxes than make changes to the production line.
Worth comparing this approach to the B-17 where a chin turret was fitted to the B-17G as a direct consequence of head on attacks in daylight.