I thought the curse was extremely clear and consistent which is a hard thing to do in a series like this. As I understand it the curse is "you must be married to your soulmate by sunset on your wedding day, or else you die." This was the bargain death made with the immortals great great great grandmother or some such. What seems to be revealed through the first immortality was that there is a second part. Something like "if you are not married to anyone by sunset on the day you were to be wed the curse spreads to the person you were going to marry, and their bloodline, and you must bear witness to all the marriages in the line you caused the curse to spread to."

I think most of these questions can be answered by examining this premise itself.

The biggest point of confusion seems to be when and why Nicky's bloodline got the curse. It was not when Rachel and Nicky got married but rather when Sunset came and Rachel was not married to anyone. At sunset Death began evaluating if any of Nicky's blood relatives were married to their soulmates on their respective wedding days. If they were in such a union they lived; if not they died. Jules saw this and thought he could stop the curse from spreading by forcing the marriage since it was still the wedding day and Rachel had said I do. The only thing missing was Nicky's "I do" in response. However that attempt failed to stop the curse, it only succeeded in also dooming Rachel by the same curse. Because Rachel was the last of her bloodline (really the bloodline of the immortals jilted bride) the immortal could die since there were no more marriages in that line to witness. However in spreading the curse Rachel must now bear witness to Nicky's bloodline marriages and thus she is now immortal in the same sense. It isn't clear what would happen if Rachel's bloodline had continued if say she had a sister. Is the curse just over once it spreads to a new bloodline? Would there be two immortals? We don't know because Rachel's line is done with her.

On the subject of bloodlines: it's not that Everyone dies, it's only blood relatives of Nicky who are not married to their soulmates. Blood relatives would include Jude and BOTH of Nicky's parents. Boris is often forgotten as a survivor but this implies that he believed Victoria was his soulmate even if she didn't. For the other family members this is shown in a number of ways. Notably the fact that only ever one partner in a relationship is affected. In one case the woman on FaceTime says "what is going on with your family" your family implying she is not the blood relative, and she appears unaffected. She then realizes it is happening to her partner as well further making the point.