Another way to find such a book is to ask in music or literature - focused forums. People who are passionate about Mozart and fiction might be able to point you to a book. It could be that a new book has been published recently that combines the allure of Mozart's night music with a fictional narrative, but it may not be widely known yet.
One such book could be 'Mozart: A Life in Fiction', which might touch on his composition of night music. It could explore how his music, like his nocturnes, was influenced by the environment and his own emotions during the night.
It shows the competitiveness in the real classical music world. Musicians are always vying for positions in top orchestras, just as depicted in the show.
It depends on the content and presentation. If it involves made-up characters and events, it's likely fiction. But if it presents real facts and experiences, it could be nonfiction.
I have no clue. It could be that someone just came up with the idea of combining the two, but there might not be a logical or established relationship between them.