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.
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.
It's highly probable. The Glee fandom is quite large and creative. Writers might be interested in exploring a storyline where Marley is pregnant as it can add a new dimension to her character. To find it, use relevant search terms like 'Glee Marley pregnant fanfiction' on search engines dedicated to fan works, and don't forget to check the tags and categories on those platforms.