He might become very withdrawn. Bell could start to isolate himself from others, especially those associated with Hestia. He may no longer trust as easily as he did before.
I'm not aware of any such 'Twilight' fanfic with that title. Fanfics in the 'Twilight' universe usually focus on the relationships, adventures, and the vampire - human - werewolf dynamics. A title like this is rather inappropriate and goes against the general themes of the original series.
After the initial shock and sadness, Twilight often becomes determined. She decides that she will not let the betrayal break her. In a story where she is abandoned in a dangerous realm, she uses her magic and wits to find her way back and also to expose those who betrayed her, showing that she is stronger than they thought.
The gods would have a variety of reactions. Some might see it as a sign of something special, perhaps a new era beginning. Others might be more cynical and think it will bring trouble. For example, Freya might be interested in the child's potential and keep a close eye on the family. Zeus, if he were still around, might offer his wisdom on raising a child with such unique parentage. And Hermes could spread the news far and wide among the gods and mortals alike.
Bella might go into a deep depression. She was so in love with Edward, and his death could shatter her world. She could become withdrawn and isolate herself from her friends and family.
Edward would likely be in a state of panic. He loves Bella deeply, so he'd probably do everything in his power to save her. He might search for a cure or a way to reverse whatever is causing her to die.
Edward would be completely devastated. He would likely go into a blind rage and seek out whoever was responsible for Bella's death. His love for her was so intense that he would stop at nothing to avenge her.
However, some human Cullens might be a bit perturbed. If a fanfiction misrepresents them or their family values in a negative way, they could feel a bit of annoyance or disappointment. For example, if a fanfiction portrays them as being cruel for no reason.