Might have more to say about this later, but Livvy passing on to full death only makes sense if you see her as a plot device motivating Ty's story. If you look at her as an independent character, then coming back to life is a more satisfying resolution.
If Livvy is just there as a prop in Ty's arc, then yes it would be good character development for him to let her go and accept the things he cannot control. However, Livvy is her own person and she doesn't want to let go either! The two of them are bound together, and both of them want to stay bound, even if it means playing with dark magic and keeping Livvy in a strange twilight state.
Livvy's existence isn't just Ty's decision. In The Lost World, Livvy thinks that she would rather be an albatross around Ty's neck than a dead sister. She wants to be alive, and if she can't be alive then she wants to be a ghost. This means that if she's going to pass on in TWP, then she will need to have her own journey of acceptance. Conversely, if she doesn't accept her own death and keeps fighting for a chance at life, then a proper resurrection could be really cathartic (since it would show Livvy finally taking control of her own existence and changing her own future).
For as much as TSC goes on about necromancy being bad, literally not one attempted resurrection has failed across the entire series. Clary brings Jace back, Lilith brings Sebastian back, Lucie brings Jesse back, and Malcolm brings Annabel back. There's usually consequences, but the resurrection itself always works. That's what I think will happen in TWP: Livvy comes back, and it causes problems, but she ultimately gets to live.
