Does a father's guardianship over his daughter, who has embraced Islam and wishes to marry a suitable man, fall away despite his repeated refusal of her marriage and his prevention of her wearing the niqab, knowing that he does not observe prayer and sometimes drinks alcohol?

Congratulations on the sister's conversion to Islam. It is obligatory to cover the face from non-mahram men, and it is forbidden for a father to prevent his daughter from wearing the niqab. If the father is an evildoer for not maintaining prayer and drinking alcohol, then according to the Shafi'i and Hanbali schools of thought, his guardianship for marriage is not valid, and guardianship transfers to the next in line among the male relatives (paternal grandfather, son, full brother, ... then the judge). If the guardian prevents his ward from marrying a suitable man whom she accepts, he is considered an oppressor (عاضل), and guardianship transfers from him to the next in line among the male relatives, then to the legitimate ruler (judge or Islamic center). It is not permissible for a woman to marry without a guardian.