Dawntrail's MSQ story is a disaster and waste of potential. [full spoilers]

[Eyrilona]

Totally agree. Minor note is that the Lala's came from the Source from the South Seas islands, specifically Aloalo. Might have been mentioned in some of the side quests. Several references to the Speaker and a giant statue, etc. I was also confused at first and also thought they meant another reflection. But now we have another mystery of how a relatively primitive culture had an artifact that allows for dimensional fusion, and the Speaker had supposedly had it for years/potentially had it passed down.

This may not be the thread for it, but we could embark on some wild speculation here...

Since the miilalas left the Source during the event of the 5th umbral calamity, but arrived prior to the 2nd, we have a situation that can not be explained by relative differences in the passing of time, because the timelines would be cross-wired... so we have to presume that the key not only allows travel between reflections, but time as well. Combine this with how in ARR we see that Tataru, before turning her hand to the crafting jobs, tried to join the arcanists guild, but gave up after her carbucle ran away from her. However, in Endwalker a side quest in Labyrinthos reveals that this same carbuncle was found still in that area several expansions later, and the researcher on the case states that it would take an unimaginably powerful arcanist to bring forth a carbuncle that could persist so long on its own, seeding the theory that Tataru may actually have a natural talent for being the most powerful arcanist in known history. And possibly not entirely relevant, but Island Sanctuary additionally shows us Tataru becoming a regular visitor to sunny islands.

And with the scions becoming the guardians of that key, these two things come together and create potential for some event, by mishap or intentional, whereby the coinkeeper of the scions (being the character most likely to open the scions' vault for business, only to be sucked into a portal opened in there by the key) is transported to the South Sea Isles in the distant past, and becomes the unwitting founder of miilala culture and the original art of arcanima on the Source? If the key then remains in that past, you would have a closed time loop which is often the only way to narratively get rid of an overly convenient time travel device...

I doubt any of that is going to happen of course, but all the little hooks and lore bits to make it work are actually there.