Well, Dumbledore has always had a special connection with Harry. When he finds out about the abuse, he would be devastated. His first instinct would be to comfort Harry. He might also reflect on his own actions and whether he could have prevented the abuse from happening. Dumbledore would then use all of his resources to make sure Harry is safe and happy. He could provide Harry with a mentor or a guide to help him through the difficult times, someone who can teach Harry how to cope with his past and move forward in a positive way.
Harry's life would likely become more stable. He might get proper care and attention, which could improve his academic performance at Hogwarts. Also, he could start to make more friends as he is no longer burdened by the secrecy and pain of the abuse. Dumbledore might arrange for him to have some kind of therapy within the wizarding world to deal with the emotional scars.
If Harry were to create a Horcrux in fanfiction, it would likely lead to a great corruption of his soul. Horcruxes are dark magic, and the act goes against all that Harry stands for in the original series. He might start to show signs of increased aggression, a detachment from his friends, and a growing attraction to the Dark Arts.
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, it could be that Dumbledore, in some fanfic versions, figures it out first and then reveals it to Harry. Dumbledore is often portrayed as having a deep understanding of magic. He might have noticed some strange magical patterns or behaviors related to Harry that led him to suspect and then confirm that Harry was a Horcrux. And then he would tell Harry about it, which is how Harry comes to know.
Perhaps his father's old wand. It has a connection to his family and his heritage. Since it's a wand, it already has a certain amount of magical energy, so it could potentially be a suitable vessel for a Horcrux. And it would be a very interesting twist on the concept of a Horcrux as wands are not a typical choice.
One consequence could be that his connection to Voldemort becomes even stronger. This might lead to him being able to sense Voldemort's actions and thoughts more clearly, but also make him more vulnerable to Voldemort's attacks on his mind. Since the Horcrux is a part of Voldemort's soul, it could also start to influence Harry's personality, making him more aggressive or moody at times.
Ginny's reaction could also involve a sense of helplessness at first. But then she'd quickly gather herself and use her resources. For instance, she might call on her friends and family for help. She'd be by Harry's side constantly, using her presence to comfort him and also using her knowledge of magic to try and ease his pain.
Yes, in some fanfiction, Harry Potter is considered a Horcrux. This concept adds an interesting twist to the original story, exploring different aspects of magic and the connection between Harry and Voldemort.