Kakashi might be initially surprised. But then, he would probably use it as a teaching moment for Naruto. He could say something like 'Naruto, stealing is not the way of a ninja.' And then give him a punishment like extra training.
The Uchiha might initially be filled with anger. Their trust in Naruto, if it was there before, would be completely shattered. They could start to plot revenge against Naruto, believing that he has wronged them. This could lead to a lot of tense moments in the fanfiction as the Uchiha gather their strength and resources.
Often, the theme of friendship between Kakashi and an Uchiha character is seen. Despite the differences between Kakashi and the Uchiha, they form strong bonds in fanfics. Also, the exploration of the Sharingan, the Uchiha's special ability, in relation to Kakashi is a common theme. Maybe Kakashi is learning more about it or helping an Uchiha with its control.
Kakashi's reaction would be complex. He is a man who has lost many people in his life. But Naruto was special. He would hold Naruto tightly, his hands trembling slightly. He might try to use some of his own chakra to keep Naruto alive a little longer, but deep down he knows it's futile. His face would show a mixture of pain, regret, and love. He would remember all the times he trained Naruto, all the lessons he taught him, and how Naruto had surpassed all expectations.
Kakashi would likely be very concerned. He would immediately assess Naruto's injuries. If they were serious, he would use his medical skills to start treating him right away. His first reaction would be to make sure Naruto was stable and out of immediate danger.
Kakashi might immediately offer a comforting presence. He could put his hand on Naruto's shoulder and just be there silently, letting Naruto know he's not alone.
Kakashi may also get angry. Angry at the people or things that caused Naruto to get those scars. This anger could fuel his determination to make Naruto stronger, so that he can better defend himself in the future. He might then intensify Naruto's training, all while having a new - found sense of protectiveness towards Naruto.
At first, Kakashi might try to distance himself from Team 7's possessiveness. He values his independence and doesn't want to feel smothered. However, as he realizes that their actions are coming from a place of deep love and loyalty, he starts to accept it more. He may even start to rely on them a little bit, for example, allowing them to help him with his paperwork or other small tasks that he usually does alone. This shows a new level of trust and closeness between them.
Some Uchiha might be suspicious of her at first. After all, she's an outsider. But others may be curious about her skills, especially if they know she's a strong ninja from her own village. They could either welcome her with open arms if they see potential benefits from her visit, or keep her at a distance if they fear she might bring trouble.