Perhaps Abuela believes that Mirabel's presence without a gift is somehow jinxing the family's magic. In the fanfiction, she might think that by imprisoning Mirabel, she can stop the negative effects she thinks Mirabel is causing. This could be related to some superstitious beliefs within the Encanto about the balance of magic and the role of each family member.
Mirabel could use her wits. Since she has a strong will and determination, she might find a weak point in the place of imprisonment. For example, if it's a locked room, she could search for a hidden key or figure out how to pick the lock.
They might be really surprised. Some characters could be flattered by the positive portrayals in the fanfiction. For example, Mirabel might be happy to see different interpretations of her story and how she's seen as a hero in some fics.
Encanto is about a magical family living in a hidden place in Colombia. Each family member has a unique gift, but the main character, Mirabel, doesn't have one. The story follows her as she tries to save the magic of their home.
Encanto is an original story. Disney's team of writers and animators came up with the concept and brought it to life on the big screen. It's not a adaptation or retelling of another story.
Indeed, Encanto is based on a story. The creators combined different imaginative concepts to bring this story to life, featuring themes of love, unity, and self-discovery within a magical setting.