Sasuke would probably be very guarded at first. He's not used to dealing with something so innocent and vulnerable. He might be suspicious of the baby's presence and think it's some sort of trap.
On the other hand, Sasuke could be very stoic about it on the surface but internally be in turmoil. He might start to analyze his own actions and whether he had missed any signs from Sakura. He could also be the type to immediately start thinking of ways to win Sakura back if he realizes he can't stand the thought of her with someone else.
Sasuke might be initially surprised. He is used to the Sakura from his past, so seeing a different, more mature future Sakura could catch him off guard.
I think young Sasuke would have a very intense reaction. He was so consumed by his hatred for Itachi at that young age. A Sasuke vs Itachi fanfiction would be like a trigger for him. He would be eager to see how the fictional fight is portrayed. If Sasuke in the fanfiction is shown as strong, it might fuel his own ego and make him strive even harder to become powerful. However, if the fanfiction shows Itachi as superior, it could enrage him. He might feel that it undermines his own goal of defeating Itachi and he might even want to find the writer to correct their 'wrong' view.
Sasuke might initially be shocked. He has always been a bit aloof and focused on his own goals. But once the shock wears off, he would probably feel a sense of protectiveness over Sakura and their unborn child. He'd start to think about how to keep them safe in the dangerous ninja world.
Sasuke would be completely shattered. His world would be turned upside down. He had always seen Naruto as a rival, but also as someone important. Naruto sacrificing himself for him would make Sasuke reevaluate his entire life. He would feel an overwhelming sense of guilt and would likely isolate himself from others. However, over time, he would start to use Naruto's sacrifice as a motivation to become a better person and to protect the things Naruto held dear. He would start to follow in Naruto's footsteps in terms of values like friendship and loyalty, and would try to make amends for his past mistakes.
Sasuke could also have a complex mix of emotions. On one hand, he has a long - standing relationship with 'Naruto' as a male friend. Now that he knows Naruto is a girl, he might start to feel a different kind of connection. He may become more protective, but also a bit confused about his own feelings towards this new - found knowledge. He might even feel a bit betrayed that Naruto has hidden this fact for so long.
Sasuke might be in shock at first. He can't believe that Naruto is dying in his arms. He may feel a great sense of guilt, thinking that he could have done more to prevent it.
Sasuke would likely be initially shocked. He's a man who has been through a lot and is used to a life of solitude and danger. But then, a sense of protectiveness would kick in. He'd become more vigilant about the safety of Sakura and their unborn child. He might also start to think about how to be a better father than his own was, in some ways, trying to break the cycle of his family's past.
Sasuke would likely be very guarded in his captivity. He would not show all his cards, even if Orochimaru tries to get information out of him. He may act cold and distant, only speaking when necessary. Inside, he's probably thinking about ways to turn the situation to his advantage. He could also be biding his time, waiting for the right moment to make his move. Given his goal of getting stronger to avenge his family, he might consider using Orochimaru's resources, but he would never fully submit. He's always aware of the risks and the fact that Orochimaru could turn on him at any moment.