If Harry realizes that he can't escape physically right away, he might start to play mind games with Voldemort. He could pretend to be more afraid than he really is, making Voldemort overconfident. Then, when the time is right, he could use this overconfidence against Voldemort to make his escape.
Harry could also try to reason with Voldemort. He might remind Voldemort of the prophecies and how killing him might not be in Voldemort's best interest. He could also try to appeal to any bit of humanity that might still be left in Voldemort, although that's a long shot.
Harry may also feel a sense of adventure. Despite the situation, he's used to facing strange and dangerous things. He might start to think about how he can use this situation to his advantage, like finding out more about his parents or the secrets of the wizarding world that Sirius might know. He could be a bit excited as well as scared. For example, he might be thinking that this could be a chance to get away from the Dursleys for good.
At first, Harry would likely be shocked and angry. He's used to being in dangerous situations, but being kidnapped by Fleur, someone he thought was an ally, would be a big betrayal. He'd probably try to fight back immediately, using his magic if he could.
Well, in some fanfictions, Voldemort might see Harry as a tool to be used against his abusers. He could use this knowledge to manipulate Harry into joining his side, thinking that he can offer protection or revenge against those who have wronged Harry.
Harry's reaction could be a mix of fear and defiance. He's faced Voldemort multiple times, so he won't be completely helpless. He may try to understand Severus' psyche, remembering that Severus has always had a somewhat ambiguous relationship with him. He might try to appeal to Severus' better nature if he believes there is one. Harry would also be thinking about how this situation affects the overall fight against the Dark Arts and Voldemort. He might see it as a setback or an opportunity, depending on how he can turn the situation around.
Harry would likely be defiant at first. He has a strong sense of justice and would not easily submit to Voldemort's punishments. For example, if Voldemort uses the Cruciatus Curse, Harry would probably try to resist the pain and not show weakness.
Voldemort might initially be furious. He's not used to being challenged in such a blatant way. He could try to destroy the Howler immediately in a fit of rage.
The wizarding community could be in shock. Many would feel that their last hope is gone. Some might lose faith and start to submit to Voldemort's rule. Others, however, could be inspired to fight even harder, believing that they need to carry on Harry's legacy.
Ginny often reacts with defiance. She's a strong - willed character. So, she might fight back against Dark Harry, both physically and verbally. She could try to escape at every opportunity, using her knowledge of magic to her advantage.
At first, Harry would likely be in a state of disbelief. He has just defeated the most evil wizard in history and now he is being treated like a criminal. He might start to question everything he has done. However, as time passes, his Gryffindor courage would kick in. He would begin to gather evidence and allies to fight against this false accusation. He would probably also be a bit resentful towards the wizarding world for not seeing the truth. He would be thinking about all the sacrifices he made during the fight against Voldemort and how ungrateful the wizarding community seems to be.
Harry would likely be defiant. He has faced Voldemort (Tom Riddle) before and he's not one to cower easily. So, he'd probably start looking for ways to escape right away, using his wits and any small magical items he might have on him.