Just a point about the Nerevarine, doesn't the Dragon-Born bit in the prophecy refer to the imperials not necessarily the he was a Dragonborn.
It's arguable. Though the term Dragonborn seems to have many different meanings.
Royal blood- Dragonborn
Born under the mark of the dragon- dragonborn
Group of people who can fight dragons- dragonborn
It's definently arguable, but I choose to believe Dragonborn means you are a child of Prophcy.
Mankar - Open the Oblivion gates (dragonborn)
Martin - destroyed the amulet (dragonborn)
Nerevarine - destroyed the heart (possible dragonborn)
Stands to reason(or speculation) all the acts leading to the events in Skyrim have been carried out by Dragonborns.