honestly when I was considering the backstory of Hydropolis it pretty much ended up being madoka magica but on a larger scale so ah I’ll try to make as much sense as I can from my less than completed playthrough and other stuff

Leucippes’ Labyrinth was where Hydropolitan ancestors lived, and the moving of its people to the middle of the sea was… partially Cetus’(or its ancestor’s) fault. I headcanon this because if the volcano, which is west of Hydropolis, was enough to wreck a continent at least some distance away from Hydropolis, then it… would’ve been world-wrecking?? idk honestly

Nerea became queen three hundred years ago, appointing Leander as Archon, and yeah he’s probably from a royal servant’s bloodline. They do share the Eye’s surveillance so Leander knows the extent of the Eye’s effect.

The volcano erupted some time later, wiping out Hydropolis, and Nerea cast Rejuvenate to turn back time

At this juncture it works a bit like Homura(as I said, I’m very influenced by PMMM), which resets everything and everyone but herself, but locally, aka the vicinity of Hydropolis, so anyone outside of that is done for, and eventually Cetus rounds those souls up. The people inside of Hydropolis doesn’t remember anything, but those who died outside of it can, and Nerea makes sure the casualties don’t affect the citizens. The Aether may’ve been lost during then too, since it’s possible to have been washed outside of the Eye zone.

So while we not bring up Leander’s shenanigans yet, the laws should indeed be put in place pretty recently, so I think by saying the ‘death and honor upon him’ may refer to the inevitable societal unrest by the constantly adding up of laws.

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now onto the shenanigans of the many Leanders

The diary(let’s just say it’s waterproof for the sake of it) of one of the Leanders ended up in the hands(limbs) of a Water Higgledie, and out of niceness, the Higgledie decides to bring it to Hydropolis for a lost-and-found session. 

Leander, as Archon, gets notified there’s a weird Higgledie hanging around the port with a weird book, goes to investigate, and finds that the book looks exactly like his diary, so he takes it, which notifies to-be-Leucothea that the guy in the neon yellow hair is the guy to take the diary to.

That Leander finds that the contents of the diary goes beyond his own, and discards his original diary during investigation. He finds that the events match up completely, and makes a note. Volcano.

The to-be-Leucothea finds the diary again, keeps returning to sender until one of the Leanders figures it out, writes it down, and tries to actively look for the Water Higgledie. He does, because by that point the Higgledie is very intrigued as to what the fuck is up. He explains the situation to the Higgledie, names the Higgledie as Leucothea, gives her the circlet so she would stand out immediately to the next Leander, and entrusts her with the diary after a final reminder to bring it to the outer seas. 

The Leander soon after tried to bring up the time loop with Nerea and seeing her reaction, adds an additional note to not tell Nerea.

Now that the ???th Leander already knows of the revelation due to his accompanied knowledge of magic and therefore recognising it as Rejuvenate, his logical next step is to find how to prolong the spell, and from then on, the entries become timelines where there are many branching notes to what could go wrong, until something goes wrong. For some of the Leanders later in the timeline, they already have a good grip on the days so they can go investigate other places and add to the big book of ‘how to survive Hydropolis’.

Meanwhile, since Leucothea has been outside of the time loop, she’s now more than 300 years old and has been becoming stronger during all that.

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TL;DR? 1. it’s suffering my dudes and 2. how the fuck haven’t both of Hydropolis’ leaders gone mad yet