Hinata would be heartbroken at first. Naruto is someone she deeply cares about. But she's also a strong - willed ninja. So she might start by trying to understand why Naruto left. She could look for clues in the village or talk to Naruto's friends to figure out his possible destination.
Hinata would likely go to great lengths to save Naruto. She might seek help from the most powerful medical ninjas in the village or even venture out to find a rare herb or a mysterious healer. She would be in a state of shock at first, but then her love for Naruto would drive her to take action.
In some fanfictions, Naruto is completely oblivious at first. He doesn't understand why Hinata is acting strange. He just goes about his day, being friendly as usual. But as Hinata's jealousy becomes more obvious, he might start to notice that something is wrong. He could be confused and try to ask Hinata what's bothering her.
Hinata might be shocked at first. She is a gentle soul, so seeing Naruto be so violent towards Sasuke would be unexpected. But then she would probably try to stop Naruto, as she doesn't like to see people fighting. She might also feel a bit guilty, thinking that her relationship with Naruto somehow caused this conflict between the two friends.
The villagers of Konoha would be divided. Some would trust Kakashi and believe there was a good reason for Naruto's death, while others would be furious. The older generation, like Tsunade if she was still around, would demand an investigation. And for Hinata, she would be the most affected. She might feel like she lost her purpose in life with Naruto gone, but then find new strength in seeking justice for him.
In a lot of Naruhina fanfiction, Naruto is indeed OP. His character has so much potential for being OP given his origin in the Naruto series. He has the Nine - Tails within him, which is an incredibly powerful source of chakra. In fanfiction, authors can take this and run with it. They can make him so strong that he can change the whole ninja world on his own. When it comes to Naruhina fanfiction, his OP nature can also be used to show how he protects Hinata, making their relationship more endearing. For instance, he might be able to take on entire armies to save her, which is a common trope in such fan - made stories.
Naruto may also try to find solutions. Despite the heartbreak, his optimistic nature might still shine through a bit. He could start to search for ways to mend the relationship with his parents, whether it's through magic in the Naruto world or by trying to prove his worth to them.
Sasuke would be deeply affected. He and Naruto had a complex relationship, from being rivals to friends. He would likely feel a sense of guilt, as if he should have been there to protect Naruto. Sakura would be heartbroken. She had always cared for Naruto, and seeing him die in such a tragic way in front of his parents would make her feel helpless. His parents, on the other hand, would be in utter despair. His father might feel like he failed as a father again, not being able to protect his son this time either.
Naruto would probably be overjoyed at first. After all, he has always longed for his parents. But with the sister, it might be a bit more complicated. He might be a bit cautious, not sure how to interact with her at first. But eventually, he would welcome her into his life with open arms.
One great 'Smart Naruto Naruhina' fanfic is 'A New Path for Naruto and Hinata'. In this fanfic, Naruto's intelligence is shown in a new light as he strategizes to protect the village and his relationship with Hinata. It has a really interesting plot that delves into how Naruto's newfound smarts can change the Naruto universe.