As for extra-dimensional parents, we know that’s not true because Krile would have the less dense aether of a shard-person and Y’shtola would’ve noticed that a long time ago.

As for Allagans, what gives me a bit of pause is that the Allagans were particularly racist against Miqo’te and G’raha’s people fled to the edge of the empire before getting absorbed by it again later and Solution Nine seems to have Miqo faces in advertisements. Also, while we know the Allagan alphabet differed slightly to the modern Eorzean one, I think it would be closer to our alphabet than the Ancient one 8,000 years before it. If the proto-alphabet is indeed Allagan, then the WoL and Scions would know IMMEDIATELY because of their familiarity with and the ubiquity of Allagan coins.

Plus I’m far more tired of Allagans than I am Ascians and I’d rather a lost Ascian be responsible than lost Allagans since that at least ties up one old plot thread.