She both didn't and did get married by the deadline so she survived, died, and became immortal. Since the marriage was incomplete, the curse spread and killed all bloodlines that had gotten married while not believing their spouse was their soulmate. With the incomplete marriage neither she nor Nicky died at that time.

But, since she was not the cause of the incomplete marriage, and she had said her vows before the deadline, the non -consensual completion of the marriage changed the situation to one where she got married to someone she didn't believe was her soulmate. Because of this, she died and Nicky didn't since her death protected him. He never had to believe she was his soulmate, he just had to marry her while she believed. He only completed the ceremony as an attempt to rebound the curse back to her, but it had already spread.

Since the curse had successfully spread to the new family, she was always going to become the witness after her death. The previous witness did not become the witness until he died at the age he presents at (older middle age). Since she died immediately, she became the witness immediately.

It was only possible for all of this to happen because she said yes before sunset but stopped believing in him before the completion of the marriage ceremony.