Two things I, as a newcomer, just noticed

[Vexander]

I believe in 1.0, prior to Dalamud's fall, Atamos (plural) attacked Aetherytes around all of Eorzea. Not sure why. It seems likely to me that since Colonial Sharlayan was abandoned that no one protected them, or replaced them when they broke. I also think the idea that they just crumbled over time holds merit as well.

Typically when characters speak of, 'Another Star,' they're speaking of one of the 13 Reflections of the Source. We're on the Source, and the 13 Reflections are other versions of the world we live on. The 13th reflection (or Star) fell to darkness and became the Void. It seems likely that is the Star Midgarsormr came from.

[Berethos]

Typically when characters speak of, 'Another Star,' they're speaking of one of the 13 Reflections of the Source.

It's generally not as specific as that - references to "this star" are generally just references to Hydaelyn as a location (and afaik references to "another star" are very rare - as in the only mentions of the reflections I can think of either call them reflections or "another world" instead of using the "star" reference, though there's likely a reference that has slipped my memory). The Ascians quite possibly mean it in that sense, given how and where they operate, but that would be by way of implication. Most others mean our Hydaelyn and only our Hydaelyn, without the other worlds being part of what they are talking about (especially since the reflections being a thing is a relatively new revelation for the player character and isn't exactly known to many).

Which means we really don't have much evidence regarding Midgardsormr's origin other than that it wasn't likely from this specific planet (and the dragons haven't exhibited any of the properties or characteristics of voidsent or other denizens of the 13th, so I'm curious as to why you believe it to be not only possible but likely).