Is it permissible for a father to force his daughter to leave her fiancé if he discovers that he drinks alcohol, and the marriage contract was concluded but not yet consummated? And is the daughter considered sinful for complying with her father's request, then regretting it and wishing to return to him after the divorce, especially as she feels she may have wronged him and herself with this decision?

The husband's immorality is a permissible reason to seek divorce. In fact, it is commendable for a woman to free herself from an immoral husband, especially if there is no hope of his rectification. The guardian may also ask his ward to do so. Your obedience to your father in separating from this young man is a good deed, and you have not wronged him or yourself. Rather, it would be an injustice to accept a husband who drinks alcohol. Do not grieve over his departure and pay no attention to the whispers of Satan. Any overwhelming feeling of desire for him is not sinful—God willing—but do not indulge it. Occupy yourself with what benefits you in your religion and your worldly life, and ask God to grant you a righteous husband. Have a good opinion of God and entrust the matter to Him, for "And if they separate, God will enrich each from His abundance."