On the first question... my understanding, from reading many different sources and from talking to many Bomber Command veterans, is that aircrews were taken by truck to their dispersals some time (up to an hour or so) before departure. The signal to start engines would be a green flare fired from the watch office (or a red flare if the operation was 'scrubbed' at the last minute). Then the bombers would start their engines and taxi around the perimeter track to the duty runway. The final 'clear for take-off' was a green Aldis light flashed from the runway controller, situated in a caravan near the downwind threshold.
Once actually airborne and on the way out a nd back there was no such thing as ATC as we now know it; everyone who was able to simply kept a good look-out! The theory was that if everyone flew the same headings out to the target there would be minimal collisions in the bomber stream (because everyone would be going the same direction). Clearly this was less effective at turning points. Exactly how many aircraft fell victom to collisions will never be known.
As for the return to base, the following is from a book written by Australian Don Charlwood, a navigator who after the war got into ATC and was, eventually, heavily involved in the selection and training of ATCs in Australia for many years. The book is called
Take-off to Touchdown: The Story of Air Traffic Control, and was published in 1967:
As they approached England on their return journey, the aircraft would split up. About fifteen miles from home, each would call its own control tower using the tower's radio call-sign - a strange assortment of call-signs, all of them distinctive to avoid confusion with others: Eggwhisk, Hotpipe, Porkchop, Dopey, Fusspot, Bluefrock and so on.
As soon as these calls were made, RAF flying control would take over. Although the flying control officer directed the operation, a girl would use the microphone - a distinctive voice among the many male voices calling for instructions.
The first aircraft to call would normally be given first to land; the succeeding aircraft would then be 'stacked' one above the other at one thousand foot intervals. All would circle the aerodrome awaiting turns to land.
Let us take as an example the well-known RAF aerodrome of Manston. The first returning bomber to call Manston control might be B for Baker. The exchange of requests and instructions would run as follows:
'Hello Bluefrock, this is Baker. Over.'
'Hello Baker, this is Bluefrock. Clear to land runway 18, QFE1002. Over.'
'Hello Bluefrock, this is Baker. Wilco. Out.'
Which meant that B for Baker was to land in the direction of 180 degrees - or to the south - and that the adjustment to the altimeter to ensure that it read the aircraft's correct height above the aerodrome was 1002 millibars. Baker's reply of 'wilco' simply meant 'will comply'.
Below them the crew of -Baker would see a widespread circle of lights surrounding not only the arodrome but extending over a mile from the landing area. In the blacked-out conditions of wartime England this 'outer circle' would show up clearly on all but the worst nights. The pilot would follow this circle round at a thousand feet until he reached the downwind side of the aerodrome. There he would call, 'Downwind', to which the tower would reply, 'Pancake', signifying 'Clear to land'. The pilot would begin descending then from a thousand feet and turn to face the runway in use, guided by a 'funnel' of lights leading to the flarepath itself. Lastly he would watch for a green light from the airfield controller, who was positioned in a van beside the runway and had an uninterrupted view of runway activity.
In the meantime the second aircraft to call would have been given No. 2 to land, the third No. 3, and so on. These numbers not only gave pilots their tun to land, but also the altitude in thousands of feet at which they were required to circle - No 2 at 2000 feet, No 3 at 3000 feet. As Baker left 1000 feet, No 2 would be instructed to commence his approach, and No 3 woud be stepped down to 2000 feet and become No 2.
Once an aircraft had landed, a system of taxiway lights at ground level guided it to its parking place.
Each bomber Group had a variation on this system, and I believe it was standardised across the entire Command in early 1944 or thereabouts. I do have a record of a 467 Squadron crew whose pilot received instruction in the "5 Group Quick Landing Scheme" in January of that year. This differed from the system that Charlwood explains by the addition of a second radio frequency - aircrft were organised in the stack on one freq and then transfered to the second one once actually in the circuit. With up to 40 bombers all arriving over the airfield at once this was to reduce frequency congestion. It was quite efficient, with an aircraft landing every minute or so. I've seen one particular night where three bombers landed inside the same minute, a remarkable effort!
So the simple answer to your question is: flares and lights only on the way out, and radios for the return. Clearly radio silence was no longer reuired once the target had been hit!