I definitely think the older guy in Judge armor looking at a tattered flag is Nerva. The blue-haired younger Garlean is probably someone brand new.
The shot of the Ancients running through the destruction of Amaurot has them more distinctly "there" and less ghostly than Emet's recreation, so I assume it's an actual flashback of the actual event.

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Wiz33
Well, Guess the Launch trailer and comments in the live letter pretty much confirmed that Emet is alive and kicking. I'm even going to go out and say he's going to have a major impact in the Endwalker story.
I don't see how you got the impression he's alive at all. We know his spirit is around in the Lifestream because of 5.3, so him being the expansion's zone narrator ala Fortemps/Lyse/Ardbert doesn't mean he's physically alive.