Am I obligated to ask my fiancée about what has been said concerning her, and would my marriage contract with her be invalid if she had committed adultery in the past?

The young woman's matter should be taken at face value, and it is not permissible to harbor ill suspicions about her without clear evidence, for Allah Almighty says: "O YOU WHO HAVE BELIEVED, AVOID MUCH [NEGATIVE] ASSUMPTION. INDEED, SOME ASSUMPTION IS SIN," and the Prophet, peace and blessings be upon him, said: "Beware of suspicion, for suspicion is the falsest of speech."

Engaging in slander against her honor and accusing her without clear evidence is considered an accusation (qadhf) that necessitates فسق (transgression/disobedience), for Allah Almighty says: "AND THOSE WHO ACCUSE CHASTE WOMEN AND THEN DO NOT PRODUCE FOUR WITNESSES - WHIP THEM EIGHTY LASHES AND NEVER ACCEPT FROM THEM TESTIMONY, AND THOSE ARE THE DEFIANTLY DISOBEDIENT." The questioner has no right to question her about this matter. If the marriage contract was concluded fulfilling its conditions (such as the guardian and witnesses), it is valid, and whatever may have transpired from her does not affect it. One should direct their thoughts away from these doubts and consider the young woman's current state.