Percy could react by ignoring Clarisse at first. He might think that if he doesn't give her a reaction, she will stop. But when that doesn't work, he could start to fight back verbally, using his quick thinking to come up with comebacks.
One common theme is self - discovery. Since Zenitsu has a complex personality, many fanfics focus on him finding out more about himself, his powers, and his place in the world. Another theme is friendship. He often forms strong bonds with other characters in fanfics, like Tanjiro and Inosuke.
Definitely not. In the manga, there's no such plotline of Zenitsu and Nezuko getting married. Their relationship is more focused on being comrades in the fight against demons rather than a romantic one leading to marriage.