I personally think that you were "born" in that carriage...but not in the sense that you suddenly popped into existence right then and there.

Remember the Ascian Emissary's (I can't spell his name) words, saying something like "When you master the Echo, you'll want to join us?" and him suggesting that we're not so different?

What if we're aetheric beings, just using the body we inhabit to further the Mothercrystals goals? Kinda like how the Ascian warriors inhabit other people's bodies to work in this world...that in the carriage, we're "awoken" into that body?
Perhaps the twins and merchant were sent by the Mothercrystal to ensure things went as planned, to make sure you are fine? That they were traveling/escorting with the person (more of a shell when you think of it, really...) before your "awakening" and knew this would happen?

I'm trying to make sense of the scene before the carriage; you're supposed to be woken to the Echo with the Starshower scene...and yet, you hear the Crystal and bathe in her light before that...What if that scene really happened (wasn't just a dream), and that "fight" with Lahabrea resulted in you "waking up" in the carriage?

And now for a crazy thing I just thought of...what if you lost that "fight", and were thrown into the carriage because of it (perhaps "saved" by the Crystal from him by inhabiting that body)? You're a rogue Ascean maybe, defecting to the Mothercrystal, and he was trying to punish you before you could do anything? That the Crystal had a contingency for your "survival" and Her plans and this is it? And now I'm thinking of more crazy theories that don't hold water lol.