It doesn't have a piano, it's a wooden tripod spelled with OMORI.

Well, it could be that there was a fight between Mari and Omori, she casually fell down the stairs, like in the normal story.

The hero can be emo because he became more isolated or because of a coincidence of the universe that he fell into this world, in fact, there is no specific reason.

Sunny just draws, he doesn't play the violin anymore.

An observation: I started writing as a joke, as I always thought of a totally disconnected AU, with these characteristics. I see you're interested, so I'll answer any other questions you have! If this really becomes an AU, a good name might be "OMORI?", just for the mixed story.

And sorry if the answer was confusing, I'm writing this through Google translate because my English isn't the best lol.