The difference between the OST and the theme song was the scope and application. OST referred to all the original soundtracks in movies, TV series, anime, or games, including theme songs, interlude songs, and ending songs. The theme song was only the opening song of the work. The theme song was a complete song, used to represent the main piece or song of the work. Therefore, it could be said that OST was a part of the theme song, but OST was not the same as the theme song.
Ryuk would probably just be amused. He is a Shinigami who has seen a lot in his existence. Fanfiction is just another strange human creation to him. He might chuckle at some of the more outlandish ideas in the fanfictions, like a character having superpowers beyond the Death Note.
Misa would likely be excited. She is a big fan of Light and the whole 'Kira' concept. In fanfiction, if there are scenarios where she has more power or a closer relationship with Light, she would be all for it. However, she might also get jealous if there are other female characters getting too close to Light in the fanfictions.
Oh, for sure! Death Note is considered a masterpiece in the manga world. Its unique concept and the way it unfolds make it a must-read. The characters' development and the moral dilemmas presented make it stand out from the crowd.