It could also be a story about the journey of two characters towards getting married. The light novel might explore their feelings, the cultural aspects of marriage in a fictional setting, and how the ring serves as a symbol of their bond. It might include various sub - plots such as family reactions, obstacles in their relationship, and how they overcome them with the help of the special ring.
Yes, it might be a novel. There could be a work titled 'Onna Spy Sophia' in the realm of novels, though it's not a widely known mainstream one. It could be a niche or indie novel.
I have no clear idea. It could be that 'Lala no kekkon' is a manga featured on Manga Rock, or maybe they are just randomly mentioned together. Without further information, it's difficult to establish a definite relationship.
The Kuchisake - onna is a Japanese supernatural entity. In the story, she is often depicted as a woman with a slit mouth. Legend has it that she was once a beautiful woman. But due to her excessive vanity or some misdeeds, her mouth was cut from ear to ear as a punishment. She now wanders, often wearing a surgical mask or covering her face, and when she approaches people and removes the covering, she asks if you think she is beautiful. If you answer no, she might harm you, and if you answer yes, she will slit your mouth like hers.