There's not enough information to know exactly who Betty is. She could be anyone - a normal person, a famous figure in a small community, or just someone with an interesting tale to tell.
We don't really know who Betty is just from 'Betty tells her story'. She might be an old lady reminiscing about her youth, a young woman sharing her recent experiences, or perhaps a character in a specific setting like a small village or a big city.
Given the little information we have, Betty could be a student talking about her school life, a mother sharing her parenting experiences, or a traveler recounting her adventures around the world.
Since we have no clue about Betty's story, it could be a love story where she met someone special and how their relationship developed. It could also be a story of her struggle, perhaps she overcame an illness or faced difficulties in her career and she's sharing how she managed to get through those tough times.
Her neighbors might listen to her. Since Miss Marple is an observant and intelligent woman, her stories are likely to be both interesting and educational. Her neighbors, who may have witnessed some of the events she mentions in a different light, would be eager to hear her perspective. They might gather around in someone's living room or in the local pub to listen to her tales, knowing that her insights could be valuable.
Her best friend Veronica would probably be one of the first to help. Veronica is wealthy and could use her resources to get the best doctors for Betty.
She might feel a sense of relief. After keeping the pain and trauma inside for so long, sharing it can be like lifting a heavy burden off her shoulders.
She might start by introducing herself and the context of her story, like where she was born or grew up.
Betty Broderick is the main subject of the story. She is the one whose 'final fury' is being focused on.
Perhaps it's a reference to a female author or filmmaker who is known for her storytelling skills. There are many women in the creative fields who are excellent at crafting and telling stories, and this could be a way of referring to one of them in a more general sense.
Maybe it involves her family history, like her relationship with her siblings. Rebekah might talk about the old days, their powers, and the feuds within the family.
Perhaps she is a family member. Grandmothers, for instance, often tell the stories of their grandchildren or other family members. She might be proud of you and want to pass on your story within the family or to friends.
I think Scout is around 8 or 9 years old when she tells the story. But it might vary depending on your interpretation of the text.