Frankly I don't know what people are getting tied up in. It hought this was pretty clear.

The key was used to evacuate the South Sea Isles in the Calamity of Ice, the era before Mhach, Amdapor and Nym.
They wound up in a pretty chill reflection that has a conspicuous resemblance to a very good game released in 2000, and used their arcanima to become useful and valued members of that society.
That reflection nearly-but-doesn't-entirely fall to gradual over-aspecting towards lightning causing ever greater amounts of sun-eclipsing thunderstorms.
This causes both the discovery and rise of electrope, which the lalafel that trace back to the South Sea Isles combine with arcanima to bring to new heights.
Things get bad enough that two of these lalafel, who still have the key, plan to use it, and team up with Preservation to do so.
Preservation turns out to suck and basically want to cause a Rejoining in all but name, so those two lalafel use the key to go back to the Source to hide both the key and their baby; by apparently sheer chance, they make this trip right as Galuf is studying the construction beneath the Skydeep Cenote, noticing that it resembles the work of the South Sea Islanders more than anything.
Things go really bad over in Lightning Land.
Twenty years afterwards on the Source, and somewhere in the low hundreds of years afterwards in Lightning Land, the events of Dawntrail happen.

What part of this are people having trouble with? Is it that Lightning Land isn't associated with a calamity, because there hasn't been a lightning one after the Calamity of Ice? Because Lightning Land didn't get rejoined; Living Memory is there, it's still standing, it just got right near the brink in a similar way to the Thirteenth and First. Perhaps it was prepped for a future Calamity by the Ascians, who are no longer around to own up to it. ...or, maybe the Ascians just weren't as good at their jobs as they claimed, and that happened by mistake. Wouldn't be the first time they accidentally nearly broke a reflection.

Maybe it's worth underlining what Y'shtola said, that nobody outside of the Ascians could possibly know what reflection we went to? I feel like some people might be leading themselves down a wrong path by deciding they do know which one it was, and then getting tangled up in the presumptions afterward.