Will Allah forgive the couple for engaging in anal intercourse before marriage, should the marriage be completed? And how does one repent if the marriage is not completed?

The fiancé is considered a stranger to his fiancée, and what happened between you is a result of negligence and boldness in transgressing the boundaries set by Allah. You have committed zina (fornication/adultery), which is one of the most heinous sins. It is obligatory for both of you to repent immediately to Allah. Repentance is not conditioned on marrying her, but rather on abandoning the sin, feeling remorse, and resolving never to return to it. If you both repent, then there is no harm in marrying her if possible. Otherwise, you must cut off your relationship with her and adhere to Allah's boundaries. You must conceal this disobedience and not inform anyone about it. If your repentance is sincere, Allah will accept it and rejoice in it.