1. By marketing the game on third-party websites like Reddit, and by being patient to let more people discover you, which will usually result in your list of followers building. Also, using accurate and relevant tags on the game page can also be pretty important.
2. That's a good question. One way is by having an active presence on itch forums and seeing who likes rhythm games, who doesn't. Possibly making some friends along the way as you participate in both related and unrelated discussions, etc.
3. If I'm understanding your question right, one way of gaining followers and interest, is by trying other people's games and leaving good feedback on them. I realize that not everyone is a game developer, but I feel there's no shortage of them on Itch, either. And I feel that sometimes, they'll return the favor - trying the games of other people who have tried their game and commented on it.