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.
Yes, in many fanfictions, Damon and Elena do get back together. Fanfiction writers often explore their relationship further and give them a second chance at love, as many fans were disappointed when they didn't end up together in the original series.
Perhaps there's a big threat to the town or to Elena specifically. Stefan, being the hero he is, comes back to protect her. In the process of fighting the danger side by side, old feelings resurface and they rekindle their relationship. It could be a vampire or some other supernatural being that forces them to rely on each other again.
Often, in fanfiction, there's a time - jump scenario. After a period of time apart, both Damon and Elena have grown and changed. Damon has become more self - less, perhaps having worked on his issues with his brother. Elena, on the other hand, has realized that her feelings for Damon were deeper than she thought. They then meet again by chance and start to rekindle their relationship. Another scenario could be that a magical or supernatural event forces them to work together. During this time, they remember all the good times they had and fall back in love.
Caroline, for example, might be more accepting as she is also a vampire. She could help Elena adjust to her new life. Damon, on the other hand, might be jealous or feel threatened that Stefan made Elena a vampire. It could change the power balance between the three of them.
Well, it's possible that Mang gets back together. But we need to know more details about the situation, like the reason for the initial split and the efforts made by both parties since then.
The story of a broken mirror being reunited usually referred to the story of two people who broke up for some reason but met again under some chance or coincidence and got back together. This kind of novel usually involved the emotional entanglement between two people and the various challenges and difficulties they faced after they got back together. Some novels would also describe the conflicts and misunderstandings between the two and how they would resolve these estrangement after they got back together.