I suspect there were an awful lot of these guys (Medics, Met-Men, Armourers, Engines, Film Unit, etc, etc, etc) who were non-aircrew and, basically, in ‘office jobs’ back at the ranch but who volunteered to go on some Ops to see how their specialisation affected the effective prosecution of the air war. I suspect, also, that many may have lied about their age, or specialisation, on the Crew Manifest – and that the Authorising Officer ‘turned a blind eye’! One of ‘our’ own Met Men (Sqn Ldr C Crichton-Miller) went on such trips to get real-time information on Night Illumination (same sort of thing that MacGowan was about). Crichton-Miller was shot down over Distre (nr Saumur, France) and killed in action. He was recorded as being “a Flight Engineer” - presumably as a means of proving (Geneva Convention, etc) he was ‘pukka’ aircrew – and not an “Agent”.
Some bright spark could write a Degree Dissertation on this sort of thing! I’ll bet there are a number of “untold stories” to be told!!