One possible plot could be that Edward, perhaps influenced by some dark magic or a misunderstanding, decides he has more in common with Harry. Maybe he sees Harry's courage and selflessness and feels drawn to that world. He leaves Bella heartbroken, and Bella then has to find a way to move on while Edward starts a new life in the wizarding world with Harry as his new friend or more.
Maybe Voldemort. If the fanfiction combines the Twilight and Harry Potter universes, Voldemort might see Edward as a threat because he is a vampire and could be allied with Harry. So he tries to drive a wedge between them and make Edward leave Harry.
For Edward and Harry's relationship, it's likely to be broken and filled with hurt. Harry might feel betrayed and angry. As for Edward and Bella's relationship, it could start off with a lot of drama. People around them might not understand or accept it quickly.
Well, in some fanfictions, the author might justify it by showing that Edward has a deeper, more primal connection with Bella. Maybe he realizes that his love for Bella is something he can't ignore, and that his relationship with Harry was more of a passing acquaintance or a misjudgment in the first place.
Fem Harry might be heartbroken. She could initially try to win Edward back, using her magical abilities in creative ways. For example, she might create a magical memory of their time together to show Edward what they had was special.
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.
It could be that there are external forces at play. Maybe there are other characters or some kind of dark secret in the fanfiction lore that forces Edward to leave Bella while she's pregnant. For example, there could be a threat from a rival group that says they'll harm Bella and the baby if Edward stays, and he thinks leaving will draw the danger away from them.
Edward might be initially shocked. He has always seen Bella as his own and this sudden change could be hard for him to accept. He could become withdrawn and moody, similar to how he was before Bella entered his life.
Edward might be initially shocked and heartbroken. He has been in love with Bella for a long time, so her leaving for Jasper would be a huge blow. He could become withdrawn and moody, brooding over what went wrong.
One possible plot twist could be that Edward realizes that Bella represents a more 'normal' life for him compared to Fem Harry. Maybe Fem Harry has some dangerous or complex magical powers that Edward fears, while Bella seems more human and easy to understand.