The author had an interesting set up with having reverse isekai incidents being a known phenomenon, but then he just jumps straight into wish fulfillment.
Kind of reminds me of Candy Bullets Can't Pierce Anything - https://mangadex.org/title/0df0ffad-e651-4364-8408-b49930e8f763/a-lollypop-or-a-bullet
Both are mermaid themed manga about child abuse.
Prez's reasoning for not telling Sora he's the trialing prince actually kind of makes sense. If their relationship becomes too awkward, it'll be harder for Prez to get Sora to fall in love with her naturally.
That confused me too. It only makes sense if they disappeared after their first platinum ranked quest. Otherwise it just looks like they decided to never return or write back home after their very first quest.