There could be various fanfictions. Maybe some that focus on alternative endings, like ones where certain characters survive who didn't in the original. For example, a fanfiction where Erwin survives and leads the Scouts in a different way.
Definitely fiction. The characters and events in Marley and Me are crafted by the writer to tell an engaging tale that may draw inspiration from real-life but is not based on actual facts.
Marley had a significant impact on the family in the 'Marley and Me' real story. His constant presence and his unique personality forced the family to adapt to his ways. For instance, his exuberant energy made them more active as they had to take him for walks and deal with his playful antics. His unconditional love also taught them about the value of loyalty and acceptance. Despite his flaws, he became a central figure in their lives, and his passing away left a huge void that made them realize just how much he had changed them and become an integral part of their family unit.
One way is to check on popular fanfiction websites. Look for ones with high ratings and a lot of reviews. For example, Archive of Our Own (AO3) has a large collection of AOT fanfictions. You can search for the most popular ones there.
Her character could develop through her relationships. If she has a best friend who is from a completely different family, like a Weasley - like family, she would be influenced by their values. She might start to show more kindness and acceptance. Also, if she faces some sort of adversity at school, like being bullied for being a Malfoy, she could either become bitter or use it as a motivation to change for the better and become a stronger person.