based on my assumption (feel free to correct me if i’m wrong, english isn’t my first language so this is based off interpretation) i think this is sort of a “third life” scenario where the 1st life is when she died as ‘the witch’, 2nd life is her when she was in the modern world but since the 1st life nielis didn’t want to be a witch anymore she didn’t have any memories of her 1st life and assumed her modern world self is the reality & her 1st life was a story, and 3rd life is the current one
No she isn't a transmigrator, her first life is when she "died" and right before her death she used her magic to change time and changed her memories to where she thinks that she's a transmigrator to make her second life easier to bear the pain she caused in her first life
Ok... So she's not a transmigrator STILL SO CONFUSING (╯°Д °)╯╧╧