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.
While I am sure this is correct for operations this would probaly not been the case at OTU's (where the majority of flying took place). Does anyone know what the procedures were at OTU's? Maybe they did go off on cross countries together but my research shows that there wre a lot of crews at different points in their training at the same airfield which points to dedicated single departures during the day and night at OTU's