Harry would definitely be furious. He's used to fighting against unfairness, and this forced reading would seem like just another form of Umbridge's tyranny. He might try to find ways to disrupt the reading sessions or at least make it clear that he's not going along with it willingly.
In the fanfic, it could have an impact on the relationships between students and teachers. Teachers who oppose Umbridge may secretly encourage students to resist this forced reading. This could strengthen the bond between like - minded students and teachers. On the other hand, students who side with Umbridge may be ostracized by their peers. Also, the forced reading might limit the students' exposure to a diverse range of magical knowledge, as they are stuck with the books Umbridge chooses, rather than exploring a wider variety on their own.
She'd probably be horrified. Fanfiction can have all sorts of wild ideas and interpretations that go against her traditional views. For example, if there are relationships in the fanfiction that she doesn't approve of or characterizations that are different from what she thinks is right, she'll be very unhappy.
Hermione might also try to turn the situation around to expose Umbridge's tyranny. She could gather evidence of Umbridge's improper behavior during the punishment process and then use it to convince the other teachers or even the headmaster that Umbridge is a bad influence in the school. This would be in line with her intelligent and problem - solving nature.
She'd be outraged. Umbridge is a very traditional and controlling character. Fanfic doesn't follow the official canon strictly, which would be a big no - no for her.
Dumbledore, if he finds out, would surely take action. He might not be able to reverse the effects of the Veritaserum immediately, but he would find a way to protect Harry from any long - term consequences. The other members of Dumbledore's Army would be in shock. They would worry about what Harry has revealed and also feel a sense of guilt for not being able to protect him from Umbridge's devious plan. Some of the more rebellious students might even start a protest against Umbridge's use of Veritaserum on a fellow student without proper authorization.
She'd be appalled. Umbridge is all about following the rules and maintaining the status quo. Fanfiction doesn't adhere to the official canon she so firmly believes in.
One possible consequence could be that she gets fired from her position. If Harry manages to expose her actions, the wizarding school or the Ministry of Magic might not tolerate such blatant abuse of power and take disciplinary action against her.
Some in the wizarding world might be appalled by Umbridge's actions. The more liberal - minded wizards and witches would likely support the creators of the fanfic and condemn Umbridge. However, there could also be those who are more conservative and might side with Umbridge, believing that such fanfics are inappropriate.
He might react with defiance. Teddy is not one to back down easily. If Umbridge tried to impose her unjust rules on him or his friends, he would likely oppose her openly. He could use his ability to change his appearance to make a statement, like turning into a wolf (in honor of his father) to show his strength and resistance.
Hermione would probably be defiant at first. She's not one to take injustice lying down.