Well, there IS one other point to make for why we're Andraste's Herald and not the Herald of someone else.
Where does the Fade rift occur? In what location is there a massive magical explosion?
The Temple of Andraste's Ashes, which is so full of Lyrium imprinted with the beliefs of Andraste's early followers that it would manifest historical figures relevant to Andrastian beliefs. Not Dalish, Dwarven, or Qunari beliefs.
Andrastian beliefs.
However it may have occurred, that stuff wasn't lightly imprinted or anything. I would say that the whole mountain itself was devoutly Andrastian. We know from Justice that a Spirit can sense something of an individual from their belongings and remains, and Andraste's remains were physically there in that Lyrium rich location and likely physically there at the center of that explosion.
So, what sort of effect do you think all of that would have if its energy was just suddenly released? Energy which has manifested people capable of speech and giving riddles, as well as capable of knowing the secrets and history of people present.
Do you think that energy would suddenly decide to be an Elven Creator, or believe itself to be the Stone of the Dwarves? I mean, sure. It probably actually is the Stone in some way, containing the imprints of Ancestors and everything, but that's not how the essence of that Lyrium infused Andrastian mountain is going to present itself.
With all of that energy flying about and the easily manipulated Fade being brought closer, that ****** Mountain is going to decide it must be Andraste herself. That's what all of the focus of attention in the mountain was centered on,l and that's what it would likely manifest.
Plus, it was a gathering of Chantry leadership that was wiped out in the blast, so that probably only adds to the Andrastian influence that this Fade Tear would have.
That's why you're the Herald of Andraste and not someone else. For a Dwarf you could probably claim that she was an Ancestral Spirit in the Stone, but it'll still be something that believes itself to be Andraste. You would just have to be a bit impressed that the Stone accepted a Human into itself.
So, that's the note I will leave off on. Whatever the hell was created in that explosion, it's probably been imprinted with whatever amount of Andraste that remained with her ashes as well as the beliefs of all of her followers that came to that mountain.