Who is to solemnize the marriage of a girl whose divorced mother is her guardian and not responsible for her, and is it permissible for her stepfather to be her guardian?

Neither the mother, nor her husband, nor the maternal uncle has guardianship (wilayah) over giving the daughter in marriage. Guardianship belongs only to the asabat (male agnate relatives). If the father is present and competent to exercise guardianship, it is not permissible for anyone else to be the guardian, even if he is negligent in his responsibilities. Guardianship transfers to the next in line only if the father is absent and cannot be reached, or if he refuses to marry his daughter to a suitable (kuf’) suitor; in such cases, guardianship transfers to the next in line among the relatives or to the Qadi (judge). The order of guardians is: the father, then the paternal grandfather, then the son, then the full brother, then the half-brother (from the father's side), then their sons, then the paternal uncles. If none of these exist, then the Qadi (judge) becomes the guardian.