How should a girl act with her father who forces her to sit alone with her fiancé weekly despite her objection and her fear of fitna, knowing that her fiancé is religious and well-mannered, and that she avoids speaking with him except when necessary?

The father's action of allowing the suitor to be in seclusion with his fiancée is a reprehensible act contrary to good manners, for the suitor is a stranger to her until the legitimate marriage contract is concluded. It is not permissible for him to be in seclusion with her or to speak with her without a mahram. The claim of needing to get acquainted is a weak one. Therefore, the father must be advised kindly. If he insists, then the suitor must be convinced that what they are doing is not permissible in Sharia, or the marriage should be finalized. Obedience to the father is not permissible in disobedience to the Creator. You should pray that Allah inspires your father to be guided.