There are three missing girls and the a brigadier spoke of 3 involved people.
There's a dead body in the shed - possibly real Alice
If the drawing is not a red herring - there's one body and two (then) alive.
Lena may well be dead as Sopie burnt her glasses in the cabin. (Though contacts or laser eye surgery are also possibilities). But she might be the dead in Switzerland person.
Sophie clearly knew all about the cabin in Switzerland when Driller was telling her she needed to go there alone for a while. I think that's where the girls were held most of the time (otherwise in basements in German married quarters). Place found in the search in the wood is part of false trail set by kidnappers to try to disguise the real locations.
Driller, Stone and Reed senior were all in Iraq as was Nadia, who was then in a more senior role to D and S, and when she saw S in 2014 there was clearly difficult history there.
Reed senior, as doctor, would have been under separate chain of command. So I think he knew (perhaps because he was a doctor) that the girl must have been abused to death by British soldiers, but may not have known who was responsible. But he arranged the private pay-offs to the family.
Was he the third man who later developed a conscience and was murdered before he exposed them? Or was he always suspicious and had come across some actual evidence and was killed to silence him?
Is Reed junior really dead?