A theme that frequently shows up is redemption. Princess Morbucks, being a bit of a villain in the original, might be on a path to change in these fanfictions, and Buttercup could play a role in that. Another theme is friendship or the possibility of it. Since they start from different sides, the idea of them becoming friends or allies is explored often. There are also stories that focus on competition between them, like who can outdo the other in a particular task or in winning over a certain character's approval.
Yes, there are some. You can start by looking on popular fanfiction websites like FanFiction.net or Archive of Our Own. Just search for 'buttercup x princess morbucks' and you'll likely find a variety of stories.
The main distinction of porn with a story genre from regular porn is the presence of a coherent narrative. Regular porn often jumps straight to the sexual action, while the story genre tries to add some kind of story framework around it. But in the end, both are adult-oriented and feature explicit sexual content.
Well, it's hard to say for sure. The concept of 'porn-like' is quite subjective. Some might consider certain novels with excessive sexual descriptions as such, but it depends on individual perspectives and cultural standards.
In many places, 18+ porn comics are considered illegal. This is because they can have a negative impact on society and violate moral and legal standards.