The impression I got from the Main Scenario is that Amaurot and everything in it, including the two dungeons, are Emet-Selch's creations. There's no actual Amaurotians involved, just simulacra shaped like them. They're real enough to be an actual, lethal threat, but they're not real - and now that Emet-Selch is gone, the faux-Amaurot will fade away over time (but a long enough time that it will always be there for gameplay purposes).
The summoning of Quetzacoatl in Amaurot features a simulated Amaurotian sacrificing his simulated life to create a simulated GF. It was likely Emet-Selch's goal to showcase the kind of selfless sacrifices his people made in the pre-Sundering days. Perhaps he had planned to show us that dungeon while he was still alive, but the opportunity never presented itself - so instead, we wound up investigating it after his death.
Emet-Selch explicitly does not need to take over a person in order to have a physical body. He does it with the clones on the Source because it's easier and more convenient. Did he do so on the First? That is never made clear. Certainly, he does not hold the lives of the First's residents in high regard - but he WAS trying to curry favor with the Warrior of Light and his allies. That could well be worth spending the extra effort to create a solid body from Aether (as the Scions themselves did), rather than co-opt some shmuck. Or some shmuck's corpse.Yeah, Emet-Selch chose that particular form to show us for some reason, since he took over some unfortunate person from the First (adding one more murder to his list) and shaped their body to his own.
He might have killed someone for his current body, but there's a lot of reasons why he might not have. And whether he did or not, the body he presented to us in his dying scene probably WAS created from nothing (or might not even have been a solid body at all), given that he cast off his flesh during the fight. Seems like a waste of effort to put it back on again, if it was even in any condition to be put on again!