My understanding is this is a Sicilian tradition.
Scungili is conch. I've never eaten it.
Bacala is female cod; staucch is male cod. Both are dried and packed in salt, so you have to soak them running water for 24 hours to make them useable for cooking. It's easier to work with frozen cod, regardless of sex, for the Sicilian Cod Stew I make during the winter months.
Today I made another traditional Sicilian Christmas Eve dish: Pasta with anchovies in a garlic and olive oil base. A good Italian Pino Grigio supplements it wonderfully.
Nope. I live in Sicily and no one I've met here has ever heard of it. One of my uncles lived in Bologna for 25 years and is my "go to guy" on Italian traditions and trivia. He says that the seven fishes thing came from one small village in the vicinity of Naples. When folks from that area moved to the US they took that culinary and holiday tradition with them. For whatever reason, their Feast Of the Seven Fishes spread among Italian Americans of all stripes as the years went by.
Definitely an Italian America thing. Definitely not a Sicilian thing though I do wish it was celebrated here. It's very, very tasty.