What is the extent of the permissibility of the aforementioned supplication and the Quranic verses used to attract beauty and the love of others, and are there other things that bequeath light and beauty to the face?

There is no basis for the aforementioned supplication or the mentioned acts. The story of the horse is a lie. Among the things that make a servant beloved to people and bestow light and beauty upon the face are: increasing good deeds and avoiding sins, for sins cause darkness in the heart and face, while obedience is light. Also, frequent prayer, especially the night vigil (Qiyam al-Layl) and sincerity in it, for it clothes its practitioner's face with light. And spreading the Hadith of the Prophet, peace and blessings be upon him, and conveying them to people, for whoever memorizes a Hadith and conveys it, the Prophet, peace and blessings be upon him, supplicated for him, "May Allah illuminate a person," meaning He clothes him with freshness, goodness, adornment, and beauty.