In respect of the Bomb Aimer discussion I have a copy of the log book of a chap whose qualification reads Air Observer Armament. He qualified in June 1941 after dropping just 48 practice bombs and firing a few rounds. The navigation side of his training took a little longer but when he graduated he had a total of 165 hours before he joined his first (Whitley) squadron. Only 20 of these hours were at night ! He goes on ops as a Navigator until he becomes the CO of a squadron when he operates as an Air Bomber until the end of the European War.
I am sure there were others like him who could alternate between the roles as required.