She might be devastated. Bella could go into a state of shock initially. Then, she might feel a deep sense of loss and loneliness. She could also be filled with anger towards the cause of her father's death.
Yes, there are some. You can start by looking on popular fanfiction websites like Archive of Our Own or FanFiction.net. Just search for the specific tag 'Edward is Bella's step dad' and you'll find various stories with different takes on this rather unique concept.
One possible plotline could be Bella initially resenting Edward as her step - dad but then gradually coming to understand him better and they form a unique bond. Another could be Edward struggling with his new role and having to deal with his own feelings for Bella in a more platonic way than in the original story.
There might be some very dark and inappropriate fanfiction out there with such a plot, but it's not something that should be promoted. Fanfiction usually focuses on more positive and ethical relationships, like the normal romantic relationships in the original story.
Yes, there might be such fanfictions out there. Fanfiction is a creative space where all kinds of wild and unique ideas can be explored. Writers often come up with unusual relationships between characters just for the sake of exploring different storylines and character dynamics.