Narcissa might be very angry with Harry's abusers. If it's the Dursleys, she could use magic to give them a taste of their own medicine, but in a more subtle way so as not to draw too much attention. For example, she might make their houseplants grow out of control or cause their food to spoil every time they're mean to Harry.
One possibility is that Narcissa would try to use her influence within the pure - blood society. She could spread the word among the more reasonable pure - blood families about Harry's situation. This might lead to some sort of support network being formed for Harry, even if it has to be done in secret to avoid Voldemort's wrath. She might also try to subtly nudge Draco into being nicer to Harry, which would be a big change in their relationship.
In the fanfic 'Malfoy's Revelation', Malfoy is paired with Harry for a project. As they spend time together, Malfoy notices some strange behaviors from Harry, like flinching at sudden movements. After some snooping around, he discovers that Harry has been abused. This new knowledge makes Malfoy more empathetic towards Harry and he tries to help him in his own way, which includes standing up for Harry against some of their mutual enemies at Hogwarts.
Well, first of all, the students would probably be shocked. His friends, like Ron and Hermione, would be extremely angry at the Dursleys. They might try to comfort Harry more often. And the school itself could start providing special resources for Harry, such as counseling or extra protection spells. Also, there could be a big change in the way the wizarding community views Harry. Instead of just seeing him as the 'Boy Who Lived', they would see him as a victim who needs support.
The other students could have a range of reactions. Some might be sympathetic and try to befriend Harry more. Others might be afraid, especially if the abuser is still around. But overall, it would create a stir in the wizarding community, and there would be a general outpouring of concern for Harry's well - being.
The abuser might face the wrath of the DA. They could be ostracized within the wizarding community. The DA members could use their influence to make sure the abuser is held accountable for their actions.
Well, if we consider the 'umbridge finds out harry was abused fanfiction', it really depends on the writer's take. One possibility is that Umbridge, being the power - hungry character she is, might try to cover it up to maintain the status quo at Hogwarts. On the other hand, she could also see it as a way to discredit Dumbledore, thinking he failed to protect Harry. There's also a chance that the discovery could lead to a change in Umbridge's relationship with Harry, from being a tormentor to an unexpected ally, though this would be a huge shift in her personality.
Their friendship would become deeper. Ron would be more protective and more attuned to Harry's emotions. There would be a new level of trust as Harry has shared such a difficult and personal experience with Ron.