I won't complain about the concept. Primordial Sea turns Kokomi into a mermaid? Sure, whatever. But timeline-wise, there's no way for the setup to make sense. The majority of people outside Vacher (and a few researchers in a quest I haven't done) had no idea about the existence of the Primordial Sea before the details were revealed in the main story quest. The issue is that if it took place after that reveal, they would have known about how it affected non-Fontanians differently because that was a key piece of the trial.
I demand historical accuracy from my porn, goddammit.