Icare9,
Thank you for the meticulous research you've carried out on the sad end of S/Ldr McKinnon's crew, but I'm afraid that exactly how they died is just another of the wartime questions which can now never be answered. Many mid-air collisions must have happened in the tightly packed bomber "streams", and a report of an aircraft being "shot-down", even if correct, might relate to any aircraft in the stream.
The German burial squads will have done their best in the conditions of the time, and the CWGC done the same at the sorting-out stage; but in the aftermath of the crash of a burning bomber (particularly with bombs on board), all sorts of misidentification must have occurred among the few remains which could be recovered. To put it bluntly: "It's a wise body that knows its own (war) grave". This is a terrible thing to have to say, but it is (almost certainly) so.
One of the "RAF" Sergeants (the F/E, Chappel) would have been a Fitter (or a Mech) (Airframe) or (Engine), who'd enlisted pre-war and so been given a 600,000 series (RAF) number. Subsequently he'd volunteered for aircrew duties, been accepted and promoted to Sergeant.
What I'm not sure about is "RAF" Sergeant Grimshaw, the AG Normally, nearly all the other aircrew trades came in as RAFVR direct entry volunteers, and been allotted a seven-digit VR number like the other four (I was 1,1##,###) - pretty well in the middle of the "spread" of them. But he is in the same series as Chappel (the "RAF" 6-digits again); it is probable that he was originally a pre-war airman in some ground trade who had later volunteered for aircrew like Chappel.
For it seems to have been the practice, during the war, for all enlistments (whether "volunteers" or "pressed men") to have been into the RAFVR (7 digits), which rather negates the 'volunteer' part of the RAFVR title. Of course, once 'in', all aircrew were "true" Volunteers for that duty - it could not have been otherwise.
It may be significant that we have no ages for the two 'RAF' men, although we have for three of the four 'RAFVR' ( and Ron Sweetlove, the fourth, with a '1,0##,###' number, would have come in slightly before me - '1,1##,###", and cannot have been much older than I [21] when he died). I assume that the two 'RAF' would be older men.
And my guess about Ron was correct - he was now the 'Obs' (Nav) of the crew.
Cheers, Danny.