Will the woman who was legitimately engaged to a man (her husband) legitimately be reunited with him in the Hereafter if she did not marry anyone else?

If the engagement is merely a promise of marriage, then the woman does not become a wife by it. However, if there is a legitimate contract with the guardian's permission and the presence of witnesses, then it is a legitimate marriage, and the woman becomes his wife. If she does not marry anyone else in this world, then she will be his wife in Paradise, God willing. As for your statement, "without taking my opinion," the divorce of a woman or the annulment of her marriage is in the husband's hand, based on His (Allah's) saying: "Unless they — the wives — forgo it or he relinquishes it in whose hand is the marriage tie." The husband is the one in whose hand is the marriage tie. If the husband divorces or approves the annulment, the divorce takes effect whether the woman consents to it or not. Otherwise, it does not take effect merely by the annulment of the guardians.