What was the point of shortening the nose (and moving the c of g backwards as a result) when fairing over the turret would have ‘civilianised’ the Lanc far more easily?

JK

The C of G has to remain within certain fore and aft limits and these will be clearly stated within the operating manuals for the aircraft, certainly in the weight and balance manual.
So if you remove or fit something to the airframe the effect of doing so has to be calculated and checked to see if you are still within the limits stated. If not then it may be possible to move heavy items(batteries for example)to different locations within the fuselage or add ballast, usually lead if bolted to the structure or bags of shingle if temporary. The Lancaster shown has also had its tail turret removed which being further away from the C of G would have had a far greater effect. The fuel transfer equipment that has been added is in the bomb bay near the C of G so would have less effect.
Then before flight the weight of the load including crew, passengers, fuel, baggage, water are all taken into account and again the positions of these within the fuselage are defined to keep within the C Of G limits.

Richard