As with most episodes of the new series, all aspects of one show do not make complete sense until a few episodes later or season's end. Not going to give anything away, that's what the spoiler section
is for, but keep in mind that while stories can stand on their own, the general direction of the season requires certain "story devices" be incorporated into each episode that follow the overall arc.
As for how they were granted immortality, the impression I got from it was that this was part of their punishment from the silent fury of the time lord. Left in different forms of suspended animation through the rest of time, but powerless to do anything with their immortality except suffer.
Not looking for a word war here, but the idea of the voice over as unnecessary is a pure matter of opinion, it should be considered with the purpose it was intended. While it's true we fought in that war, it wasn't on our home soil, we didn't have the bombings and we didn't have the losses Britain did. In the UK Rememberance Day is their version of Veteran's Day or more current comparison would be the memorial's that take place on Sept 11th.