Definitely fiction. Downton Abbey creates a fictional world with made-up characters and events, although it may draw inspiration from historical periods and social contexts.
Since I don't know the specific story, it's difficult to say exactly who Mrs Wilson is. She could be an ordinary woman with an extraordinary story, or she might be someone well - known in a particular community or field.
Yes, Dowton Abbey can be classified as a novel of manners. It showcases the social norms, etiquette, and behaviors of the characters within a specific social setting.