Hiya, I don't want to spoil things so I'll only answer a few parts:
- You'll potentially learn more about some of those characters, but not everything. Their stories are more of a slow burn across subsequent chapters as they'll be around Matt a lot longer.
- You'll see more of the mermaids in the full game, that's correct!
- The Christmas demo is segments taken from the full game and delivered in a separate experience so people had something to play while waiting.