I figured I may as well post another reply towards you, while we wait for Dragonized. Maybe that will shed light on what she had to do in order to bring Durnehviir back. Considering in the Dawnguard DLC, he stated that he could not fully return to Tamriel, or his strength would diminish, until he was no more, I feel it would be possible for an Elder Scroll to bring him back, without the setback of this momentary return. And as the Aldmeri Dominion had defeated the Imperials, and White-Gold Tower contains the library of which most Scrolls are kept, the Moth Priests(keepers of the Scrolls, if I'm correct) agreed, considering they surely would not want the Altmer to seize Skyrim, because then the Empire and Priests would be more or less done for.