Think you mean dorsal not ventral as the Lancaster's turrets were front, back and top
AIUI RL was a bit more complicated than that
Like a lot of bomber designs, the Lancaster was originally intended to have "all-round armament"... in this case
front (aka nose) - 2 guns
rear - 4
top (aka dorsal aka mid-upper) 2
bottom (aka ventral) 2
all guns originally the RAF standard .303. ~3000 round per gun at the rear , ~ 1000 on the others, which shows something I suppose.
The ventral turret was pretty soon deleted (both in deployed aircraft and production)
for the very reasons we have been discussing
but the turret ring remained. In 42 that ring became the mounting for the H2S ground mapping radar
Ninja-ed by PLP
When the Germans began using "from below" attacks",some squadrons ... especially the Canadians ..ditched the H2S for adhoc .50 calibre guns.
At about the same time new rear turrets were designed with 2 x.50 (about the same weight was 4x.303)
In practice, the new turrets were
not more effective as weapons
but some (especially the Rose Bros. design) had better visibility and was easier to escape from.
Few flew in WW2 though it was standard post war
As to electronic systems,
The RAF fitted active tail warning radar but had to withdraw it when they discovered the Nazis were homing on its transmissions.
Similarly, they did try automatic gun aiming systems for the Rear turret, but abandoned the "auto fire" since the IFF element never worked reliably.
However several other Radar gun sights were tried ... all with little use in WW2 but becoming standard in the postwar Lincoln.