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 He could become withdrawn and moody, brooding over what went wrong.
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.
He might become withdrawn at first. Since he's used to being in control and having a certain dynamic in their relationship, Bella's anger could shock him into pulling back. He could go off on his own for a while to think about what he did wrong.
Some fans might be really disappointed. Edward and Bella's relationship is a central part of the Twilight saga, and seeing Edward ignore Bella goes against the idea of their epic love. They would probably feel sad that the loving Edward they know from the original story is acting this way.
She might be completely devastated. Bella has always been deeply in love with Edward, so this sudden abandonment could shatter her world. She could be left in a state of shock, unable to really process what has just happened.
Hermione would probably be very curious. She'd want to study Edward, find out how his vampire abilities work. She might compare it to the magic she knows. Ron, on the other hand, would be a bit more wary. He'd see Edward as a potential threat at first. Harry would be more open - minded. He's used to strange things, so he'd likely try to understand Edward.
The FMA manga ended with a powerful conclusion. All the loose ends were tied up, and the characters' journeys came to a meaningful conclusion. It left fans with a mix of emotions - happiness for the characters' growth and a bit of sadness to say goodbye.
I have no definite idea. October 3 for the FMA manga might be the date of a significant battle, a character's transformation, or the introduction of a crucial element. Without further information, it's tough to pin down its significance precisely.