Sweet fuck, do people still think there's a story to tell explaining Capaldi's appearance in Day of the Doctor? There isn't anymore than we need it explained what Eccleston or any of the first Eight were doing prior to showing up. If anything, the interesting story is why is there two Sylvester McCoys?
RTD explained Capaldi's character in Children of Earth is a direct descendant of his character from Fires of Pompeii, and that the murder/suicide we see in Children of Earth is the timeline correcting itself because the family was supposed to be killed in Pompeii. So, for two thousand years there was a family line existing that shouldn't have.
No, it makes no sense, but I like it better that Moffat's anti-climactic explanation for why the Doctor took on the Capaldi form.