>>536771339
>lack of explanation on whether non-Fontainians could reproduce with Fontainians prior to becoming humans
This is actually explained by Neuvillette. When couples pray at the Fountain of Lucine, they conduct a ritual that creates new Oceanids waiting to be born as Fontainians in the coming months (i.e. specific couples don't have to pray at the Fountain, but as long as couples continue to pray in general, babies will keep being born).
Now, the game didn't go into details for obvious reasons. The Oceanids likely just spawn in the womb of the mother and take on a human shape therein. The idea is a little more freaky in mixed couples where the mother is not Fontainian.