Danny,
"It can't be true that they put the poor devils inside, in total darkness, then padlocked them in (what happened when they lost the key ?". I rather suspect that if it was done, it was done for the safety of those in the Bomb bay. It did strike me, years later, that loading ground personnel in the Bomb bay at all seemed "peculiar". The Lancaster had plenty of room for a couple of ground crew bods to enjoy the pre op air test, within the fuselage. However, I can only recount what I was told, and have no way of clarifying what he told me. Perhaps my father in law was a bit of a "wind up merchant", he certainly hooked me if that was the case.
Smudge
(as mystified as you)