Elena would likely be angry. She has her own agency and doesn't like being controlled. If Stefan kidnapped her, she would fight back. She might use her knowledge of the supernatural world and her relationships with other characters to try to escape. For example, she could try to call out to Damon telepathically if they have some sort of connection in the story.
She may also feel betrayed. Stefan is someone she loves and trusts. If he kidnaps her, it shatters that trust. Elena could go through a range of emotions from sadness to determination to break free and make Stefan realize what he's done wrong.
Stefan would be extremely worried and angry. As Elena's other love interest, he has a deep connection with her. He would immediately start looking for her, using all his vampire skills and resources. He would probably suspect Damon right away, given their history of rivalry over Elena.
Elena would probably be terrified at first. She might try to fight back or escape immediately. After all, Kai has a reputation for being dangerous.
She could be terrified at first. After all, Klaus is a very powerful and dangerous vampire. But Elena is also a strong - willed character. So she might quickly gather her courage and start looking for ways to escape.
Damon might initially be shocked and angry. He has always had feelings for Elena, and seeing Stefan marry both her and Caroline could make him feel betrayed. He might lash out and try to cause trouble for their relationship at first.
One possibility is that Stefan is under some dark spell or influenced by an evil force. Maybe he kidnaps Elena to protect her from a greater danger that he believes only he can handle, but his actions are misinterpreted. For example, there could be a new vampire hunter in town who has a special weapon that can kill vampires instantly, and Stefan thinks the only way to keep Elena safe is to take her to a hidden location.
Elena would probably be terrified at first. She's a human in a world full of powerful supernatural beings. Being kidnapped by Damon would mean she's in a very vulnerable position. She might initially try to fight back or escape, using any opportunity that presents itself.
Caroline would probably be really happy. She's always been a big supporter of Stefan and Elena's relationship. She'd be the one planning get - togethers and trying to make sure they have time alone to reconnect.
Caroline would probably be overjoyed and very helpful. She might offer to babysit all the time and give parenting advice based on her own experiences or what she's read. Damon, on the other hand, might be a bit more cynical at first, but eventually grow to love the baby and become very protective.
Their relationship would likely become more intense. Stefan would be even more protective of Elena, which could lead to some tension if Elena feels smothered. But overall, they'd be closer as they're sharing this new experience.
They would become more protective of each other. Stefan, being the protective vampire, would be constantly on guard to keep Elena and the unborn baby safe. Elena, on the other hand, would be more nurturing towards Stefan, seeing him as the father of her child.