Glory to Ka!
This all seems like much ado about nothing to me. The obvious answer is that the Progenitor Myth and the Elegy of the First World are both myths, which share a common motif of three dragons creating reality and dragonkind. There are probably many more such myths throughout the world, or worlds if you’re doing a multiversal campaign. There’s no need for there to be one right answer, especially in Eberron where the gods are less overt than in a lot of other D&D settings. Nobody knows what the real truth is, just like in real life.
Me? I prefer the Dawn War to be my home game settings’ true history, but I also enjoy having the First World be the dragons’ creation myth and by extension, the dragons’ justification for their actions in subjugating other creatures to their will.
Well, the chromatic dragons anyway.