1. The mermaid quest is in the game. She's found in a house in the north part of Flooded Colony. But there's a character (Dollmaker) in East Lampcrop Borderwoods you need to interact with first for her to show up there.
2. The next part of Squonk and Annie's story is not in game yet.
3. If you've already united all 3 Gremlins, then you can't see the first one anymore. (I'll add a way to see it again in a future update.)