I think "once" pulls away from the imperfective aspect of жить. "Once" is a very messy word to translate as it can be implicit in the verbal aspect or as an adverb. As far as whether or not this should be accepted, I'm not sure. When I think about how your sentence would be translated back into Russian, it seems problematic. That's my two cents. Maybe just two kopeks. :)
'before' is ambiguous, as used without a comma after it, it could refer to where they lived before they were in Siberia.
Does this mean that they formerly lived in Siberia, or does this speak of the time before they lived in Siberia?