Yes, they are relevant. The OVA (Original Video Animation) of Black Butler often contains side stories or additional content that is related to the overall world and characters of the original story. It can provide more depth to the existing characters, their relationships, and the setting of the Black Butler universe.
The OVA contribute by expanding the world - building. We get to see more of the Victorian era setting, the different social classes and how they interact, which is relevant to the original story's backdrop.
The Black Rock Shooter OVA mainly focuses on the relationship between two girls. One girl is in a rather tough situation at school, facing bullying and feeling isolated. Black Rock Shooter, who exists in another dimension, seems to be connected to her emotions. As the story progresses, the battles that Black Rock Shooter has in her world are intertwined with the emotional struggles of the girl in the real world. Eventually, it shows how the power of friendship and facing one's inner self can overcome difficulties.
Definitely not. The Black Butler series has a set of established themes such as the supernatural elements, the Victorian - era setting, and the complex character relationships. The idea of a 'leashed slave story' is completely foreign to the official canon and is likely a disturbing creation outside of the proper series. It would go against the overall tone and moral framework of the official work.
In the Black Rock Shooter OVA story, the girl in the normal world is a main character. She has to endure the hardships at school, such as being ostracized. This girl's story is important as it sets the emotional context. Black Rock Shooter is another main character. She exists in a different realm and is constantly in battles. Her battles are a reflection of the girl's internal struggles, and together they form the core of the story.
There is indeed a 'Black Butler' visual novel. It features the same dark and alluring art style as the anime and manga. The visual novel might include different storylines, some of which could be exclusive to this format, providing a new perspective on the 'Black Butler' world. It often has a strong focus on character development and the complex relationships within the story, which is great for fans who want to explore more about the characters.