The one unexplained point....
Stellan's character is an anomaly amongst his obviously British comrades in that his accent is non-British (Stellan's unchanged Swedish) yet this is left unexplained. Weirder still Sam Reid does a completely different accent as a younger version of the same character. I'm guessing that's just him making a mess of the accent...sounded a bit like a slightly strangled German accent.
It gets weirder because FINLAY never existed and is actually a composite of a couple of British soldiers. So why the accent/foreign casting?