Adam (#16),
The answer lies in your word "eventual". In 1940 I was certainly enlisted as an ACH/GD (u/t Pilot/Air Observer) at their option - and nothing else. After "Reception" (2 weeks) and "Initial Training Wing" (6 weeks) together, the usual train of events was: some of us were "selected" (with a pin?) to try as a pilot. If we failed, they retrained us as "Air Observers" (ie "Navigators", but the old "Observers" jealously hung on to their "O" brevets - the "Flying **********" - instead of the new "N"s). Presumably the rest of us were sent direct to the Air Navigation Schools.
Then the more complicated multi-engined things needed a "Signaller", these were recruited from the ground signaller trades (just as the later Flight Engineers came from ground engineers). They also needed Air Gunners, for all the turrets coming into use. First idea, give your new Signallers a month's Course in gunnery. The Wireless Operator/Air Gunner (Wop/AG) was born. He wore an "AG" brevet (and, if an airman, the W/Op badge on his sleeve).
Later, the turrets multiplied: a Lancaster crew would have three turrets but only needed one signaller: it is clearly wasteful to use Wop/AGs as just gunners, enter the "Straight Air Gunner", who was that and nothing else, he only had the "AG" brevet.
On completion of training, a small proportion of pilots and navs (1/5, at a guess), and much fewer number of other trades would be commissioned. The rest were promoted to Sergeant (and were much better off).
All this was "over my head", as a S/E (and nearly all single seat) Pilot I bumbled along quite happily as my own Nav/Wop/Signaller/factotum (had a Rear Gunner, but had no confidence that he would hit anything except our own tail); I felt no loss.
No idea about "Bomb Aimers", "Air Electronics Officers", and all the others who sported brevets by the end of the war: I just lived the simple life in India/Burma.
(The Pilot "Rejects" from the American Schools [Arnold and BFTS] were sent back to Canada for retraining, perhaps harrym, who trained there, may be able to show some light on their fate).
This has a bearing on the case of (#14) Warmtoast's sister's beau John (RIP): ..."It was the time of the blitz"... So, that was the '40/'41 winter, had he even got into uniform then ? (but in #12 you say ... "photo dates from 1942"...). Still don't see why he would want to wear "civvies". I was sporting wings and stripes in March, 1942 (Was it even permissable for Servicemen to wear plain clothes in wartime in Britain ? - How could the Police, or the M.P.s on Railway stations check your Leave Pass ? More likely to chuck you in clink as a Deserter - or at least AWOL.
You would be so much more of a "babe magnet", too (with the "white flash" * up - quite the "intrepid aviator"), do we know John's D.o.B. even approximately ?
Note *: The good folk of Temora must've been stupid beyond belief to fall for that one. Hope they were swiftly disabused !
Trust you can follow this, Danny.