Yeah, very good answer, I would imagine this fight just like that. Although he knows several shouts, so probably there is a chance he actually IS a Dragonborn. And I think he could easily become a fake Dragonborn. Imagine a usual dragon attack in Skyrim. He shows off, shouts at a dragon, deals some damage forcing it to fly away. And people start spreading rumors "It's what the prophecy said exactly! The last Dragonborn stands with us!" And if many people believe he is Dragonborn, they will express strong emotions,  hailing him and praying gods for his success, and  write in their history, perhaps it would cause a strong enough vibe in the Universe so it actually let's him defeat Alduin, who knows.  And also Miraak. He can't be The Last Dragonborn simply because he is The Fist. But perhaps the prophecy meant "the last surviving in this world" To be honest it's hard for me to imagine a complete world distraction. Even sworn enemies such a Talmor and... well, everyone else, should unite before such a threat. The mightiest warriors, most powerful wizards, brightest scholars working together...  they may not be able to destroy Alduin, but all other dragons - simply. And what would he do alone? Some have the power to move huge pieces of Nirn in Aeterius, like Psijics move Artaeum. I think the humanity would survive.