Is it permissible for the wife to continue in a tahleel marriage after changing her intention and the intention of the second husband from tahleel to the desire for a permanent marriage, especially since the contract was valid and officially declared, and how can one repent to Allah in this situation?

The tahleel marriage (marriage for the purpose of making a divorced woman lawful for her previous husband) is null and void, and invalid, regardless of whether the husband desires it or not. The couple must be separated due to the Prophet's (peace be upon him) saying: "Allah has cursed the one who makes a woman lawful (muhallil) and the one for whom she is made lawful (muhallal lahu)." It is not permissible for the woman to remain with the man because he is a stranger to her, and the husband's current desire to keep her is worthless except with a valid contract. The man must divorce her with a single pronouncement to annul this corrupt contract. If he wishes to marry her, a new contract is necessary, fulfilling the conditions of a valid marriage, such as the guardian's consent and the presence of witnesses.