These are two VERY different questions.
The references to the Roman invasion serve several purposes. First, they allow for still more accounts of the horrors (and people dying of disease, etc.). Second, they remind both characters and readers that the continent has been invaded repeatedly. Third, they establish parallels between the (fallen) Roman empire and the British empire.
As for narrative voices within voices, it creates a sense of psychological distance, and gives the impression that we don't really know what's going on there (as well as creating simple variety).