I get that, i don't remember that part of their statement clearly. It's not like my guess has any solid proof to sustain it. There's not much evidence, other than the fact Oscar clearly has a connection to the sea, to point at how he came to be. Being from a poor family in the middle of nowhere northeast Brasil seems like a very backwards spot for Poseidon to decide to pick and make a new minotaur. Also, why would he even choose to make a new one after what happened to the other? I don't think Poseidon would be frolicking in Brasil's country side as his first pick, unless there was something required that could only be found in Drowned Tapir, or it was purely an accident of fate.
I belive it was mostly an accident of fate, but Pedro did say that Downed Tapir lacked the signs of big human cities that usualy drove away the mythicals and magical phenomena as a whole so that might be it. And i don't think that Poseidon had any intentions to "make" a new minotaure since he descended to free one not to make another, but as i said he most likely lost sanity and had his fun without giving it that much thought.
And if you are curious about the scene of the juges of Hades, i think it was in chapter 16 when Asterion when to the rooftop to relax.
I really take issue with loss of sanity as an explanation for people to fuck. That's not something that just accidentally happens, specially for who is penetrating. It still doesn't explains why he'd end up in Brasil's interior. That's nowhere near Greece, nor some coastal village by the sea. They may be gods, but from what we've seen they are physical beings. I don't think they could just jump from and back to mount olympus to anywhere in the world, other than Hermes.
Poseidon's descent is a fact as stated by Hermes, yes him ending up in Downed Tapir is merely a speculation to defend my therory but going from Olymus to the mortal world is something that Poseidon is fully capable off and Olympian gods have been doing it since the beginning of time if we belive the myths. As for his actions while in his lost state it might be easier that it looks; both Zeus and Poseidon are known for their affairs with mortals with Zeus being the most famous one but Poseidon did have his share, so him "getting into character" and ending up having an affair isn't that out of his dictionnary. Perhaps he was playing as a wandering person and suddenly the fog took over his mind and idk saw Oscar's mother and you can guess the rest. And in the topic of Downed tapir being nowhere near Greece, nor some coastal village by the sea, personnaly i never payed attention to that so i am taking your word for it, we know that Poseidon was in the mortal realm for 30 years and Oscar is 21 so between the descent and the affair we have 8 years and if we assume that Poseidon started his investigations from Crete i think that 8 years is more than enough to go to Downed Tapir.