This may be a "don't think about it too hard" issue, but if the planet was divided into shards, what does that mean for the universe around each shard? In the trailer (and on the Shadowbringers loading screen), we see that after returning the darkness to the First there are stars in the sky. That means either one of two things: either the planet is enveloped in some sort of field so that only its shards exist out of "phase" with the rest of the universe (but you can still see the rest of the universe out there), or the entire universe was "sharded" into different alternate timelines as well.

If it's the former (only the planet is affected), then the question would be what happens when someone from outside the planet crosses the "field" -- do they just end up on the Source automatically and it's like the shards don't exist, or do they get automatically "sharded" into 14 copies? If someone from one of the non-Source shards were to travel to space, would they eventually cross the field and rejoin the "source" universe or would they just basically cease to exist once they exit the affected area? (Is it even possible for inhabitants of Hydaelyn to leave the planet and so be disconnected from its aether, or is the entire universe similarly full of aether?)

If it's the latter (alternate universes), then it would mean that Midgardsormr would actually exist in all 14 alternate timelines, and so he could decide independently to go to the planet in each timeline. The evolution from that point would still be different obviously.

I guess another possibility (aside all the above), since we know the Crystal Tower is the key to cross shards, is that the Allagans may have connected to the First themselves in the distant past. It could be that some experiments from Azys Lla might have been sent over (or made it over accidentally).

In any case, there's definitely a lot we don't know at this point about how all this works.