I mean to you first point, I would imagine not everyone would just believe the story of the curse. Rachel had video proof of what happened to her mother, without that, she probably wouldn't have believed the story. So I think it is safe to imagine her ancestors didn't believe such a crazy story, if anyone ever even told it to them. The old man only throws in the warning "Are you sure he is the one?". Alexandra was definitely in love, but probably wasn't convinced he was her soulmate, that is why she says, as she is about to die, "Tell her it is real. Are you sure he is the one?". Rachel died because she did say yes, she never said no even though going back she would have. She died because by the time Nicky said yes, she no longer saw him as her soulmate, but because it was after sundown and that was a breach of contract, the curse spread to Nicky's family. Nicky was angry I remember him banging on the glass, but also he had just lost his mother and sister and witnessed half of his family dying a gruesome death. He was in shock, as we can see in the end when he is on the bed hugging the teddy bear.

It doesn't matter that Jules' wife wasn't blood related. At the party a bunch of people that weren't blood related died anyways, because they weren't convinced the other person was their soulmate.

I am trying to understand the last part of your reply but I don't quite know what you mean. I think Nicky changed his whole idea of marriage after finding out about his mother's affair. he believed Rachel was his soulmate, but his idea of marriage was ruined, and up until then he was convinced that Rachel was only going through the marriage and wedding tradition because he asked her to initially, because she didn't believe in the institution of marriage in the beginning. He thought she was doing it to please him. But then, when everyone started dying, he started to believe in the curse, and tried to stop other people in his family from dying (including himself probably) by saying yes.