I've now put it 15.6 hours in the game trying to 100% and I'm having one single gamebreaking bug keeping me from progressing so far.
While trading the fishhook for the necklace the game freaks out a bit and i never actually get the necklace, the whole scene plays relatively normally but when i tried to go back to the mermaid the her dialogue was still as if i didn't have the necklace.
I was also having an issue where after getting the ocarina, i talked to marin and she her dialogue for teaching me the song. Though this wasn’t gamebreaking as i was able to get her to teach me the song when she is in the animal village.