I think this video provides some solid evidence that the ceremony is intended to encompass Bree's death.
When Bree mentions her "counselor", she says that she "knows all about the pressures of the ceremony" and "wants to help." In that situation, the most helpful person would presumably be one who had played Bree's role in the ceremony before. But Bree doesn't mention that Lucy had performed that role, just that she "knows all about the pressures", even though logically Bree would have been most comforted by someone who had done it before and so if Lucy had played that role in the ceremony Bree would obviously have mentioned it.
Since the person Bree's parents and their friends decided would be most helpful hasn't performed that role in the ceremony, it stands to reason that no person who has performed that role is available to help. We know the ceremony is infrequent, but it has occurred before. Furthermore, the idea that Lucy "knows all about" things would indicate that she had been present at such a ceremony, is in the Order of [Denderah?], etc., and Lucy isn't
that much older, so anyone who had played Bree's role would certainly still be around - unless that were not possible. Therefore, the person who plays the role Bree is to play is either sacrificed or for some other reason does not survive the ceremony.
Also, notice Bree's tone and expression when she says she isn't scared anymore. She is not making that statement in a relieved manner, like someone who just found out that a hard test they have coming up will be open-book, or someone who isn't going to have to get a root canal after all. She sounds flat and resigned to her fate. The tone of voice is almost like someone with a terminal illness who has come to terms with the fact that they will soon die; she's not scared anymore not because things won't be as bad as she thought, but because she's accepted how bad they will be.
I don't know for sure that Bree is aware she will die (nor have I decided whether I think her parents know) but I think she has to have an inkling that something big will happen - her beliefs may be that the person who dies in this ceremony ascends instantly to the afterlife or to some "next level" of existence. I doubt Daniel is fully aware (he would do more if he were) but he is afraid he'll lose touch with her if she goes through with the ceremony - he just doesn't realize how completely and permanently.
On a side note, I think Bree is plenty mad about Daniel's stalking video - she has the same flat, measured tone of voice, with just a hint of icy anger behind it, that girls have used for millennia to let their boyfriends know how much they're in trouble
On edit: I do not believe that Bree
will die in the ceremony, just that if the ceremony were to go as planned, she
would die. Something will happen, likely involving Daniel IMO, that either prevents the ceremony, prevents Bree from taking her assigned role in it, or saves Bree from her fate at the last moment.