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.