Anorak point about military and DNA.
The policy of collecting samples from personnel sent to the more dangerous operational theatres was introduced in 2009. And it's an actual sample that is retained, the analysis of DNA is performed only when there is need. So there is no time to be saved by using Sam's military sample. Though it is possible the sample was switched - but that would make a difference only if if was switched with the genuine corpse father. Or the shed girl sample/results were interfered with, to match real Sam. Or both.
This would require access to confidential medical records, or active collusion of several staff (who tested the DNA samples, it would be too noticeable if no tests were done) and strong possibility that German co-investigators would notice.
Now, because we have seen so much of Gemma, we want to believe she could still be alive (much as Sam wanted to believe 'Alice' was Alice, but obvs not as intense) and are ready to buy in to the idea that the garrison is riddled with criminality.
But it could all be a red herring, and it is simply the case that Alice died in circumstances as yet unknown and body was disposed of in the fire, arranged by Driller and the much-groomed Sophie.
What's on my mind is whether the incident with the Iraqi girl took place in 1991 (as we are led to believe by date of flashback) or if there has been a subsequent tour (after 2009 when DNA sampling began) when they all went out again, including Sam.