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.
Edward would be consumed with grief and guilt. He might blame himself for not being there to protect Bella. He would then use all his vampire powers and resources to track down the perpetrator. He would be relentless in his pursuit, and his anger would be a terrifying force. His family would probably have to hold him back from doing something rash at first, but eventually they would join him in seeking justice for Bella. He would also be in a state of deep emotional turmoil, constantly replaying his memories of Bella and wishing he could have changed the outcome.
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 She could become withdrawn and isolate herself from her friends and family.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.
Together, they would likely try to make Renesmee understand that their relationship is special and that her presence is also special. They might give her a hug or some form of affectionate gesture to show that they are a family unit and that her walking in is just a normal part of family life, even if it was an intrusion on their private time.
There are other fans who are interested in exploring different storylines. They see it as a creative way to expand the Twilight universe. They might enjoy seeing how the characters would react in such a difficult situation, even if it's not in line with the original story.
Edward would probably be very confused at first. He would try to understand why Bella would leave him for Rosalie. He might start to analyze all their past interactions, looking for signs that he missed. Eventually, he could become angry, not at Bella directly, but at the situation. He might also feel a sense of betrayal, not just from Bella but also from Rosalie in a way, since Rosalie is 'taking' Bella away from him.
Bella would be heartbroken. She has put so much trust in Edward, and this act of cheating would make her question everything. She might become angry and lash out at both Edward and Tanya. She could also become very sad and isolate herself from others.
She might be devastated. Bella was deeply in love with Edward, so his leaving would be a huge blow. She could cry for days and feel like her world has ended.
Perhaps in the fanfiction, there's a misunderstanding within the werewolf pack. Some members believe Bella is a threat to their territory or secrets. They attack her without fully understanding the situation, and unfortunately, she is beaten to death. This could lead to a huge rift between the vampires and the werewolves, as Edward and his family would seek justice for Bella.
Bella might be extremely embarrassed. She would probably blush and stutter, trying to come up with an explanation for their situation. Since she loves her father and doesn't want to disappoint him, she'd be very flustered.
Some fans would be outraged. Edward and Bella's relationship is iconic, and any deviation from their loyalty to each other would be unacceptable to them.