Short answers, pulled from the story.
Mary and the Witch's Flower was released theatrically in Japan on the 8th of July 2017. The film was distributed by Toho and later released in the United States and Canada on the 19th of January 2018.
Hiromasa Yonebayashi directed Mary and the Witch's Flower and Studio Ponoc produced the film. This project marked the studio's first feature film following the dissolution of Studio Ghibli.
The flower that grants Mary the power to become a witch is called fly-by-night. Tib and Gib, the cats of Peter, found this glowing flower which allowed Mary to attend Endor College.
Madam Mumblechook and Doctor Dee wanted Mary at Endor College because they believed her red hair indicated she was the most powerful type of witch. They sought to use her natural red hair and the fly-by-night flower for dangerous transformation experiments.
Mary and the Witch's Flower grossed $42.2 million worldwide with $27.6 million earned in Japan alone. The film was released on home video formats including DVD, Blu-ray, and 4K Ultra HD by Walt Disney Japan and Universal Pictures Home Entertainment.