The narrator can be sort of a problem like that. But the way I see it, the whole series is really more a historian looking back and drawing connections throughout different points in the past. So when it decides to comment on a particular point by saying something like "X character would never return to meet Y character again after going to the battle of Z," its not really a spoiler, as much as the intent is to present the information the same way that historians look back at our own history.