At first it seems that Addy showing up is unrelated to Bella being smart - then you realize that the Volturi only met Addy because Bella informed then about Joham and hybrids, which caused Addy to join them.
Allirea's power explains why hybrids are completely unknown.
Kind of a meta example, but I've heard complaints about Luminosity's Bella being too... clinical, too formal in narration and while I don't mind it, it is kinda true. However, I also noticed the narration gets less formal once she's a vampire. This makes sense if you realize, based on the prologue, that she's telling the story from memory after the Volturi attack on La Push. The reason the human!Bella section is more formal is because she's telling it from vague human memory and memorized journal entries, whereas the vampire section is more natural.
Jane is using memories of her Turning as a weapon. It is described as burning pain, and even though the text implies the pain is a memory of being burned as a witch before being Turned, she would have fallen unconscious from smoke inhalation long before she would have felt the flames, so she could not remember that. And being Turned is described as the worst pain humans or vampires could ever feel. Who wouldn't weaponize that?
Cody, upon introducing himself to Elspeth, comments that he's possibly the least interesting half-vampire in existence. This is highly ironic after learning about Allirea, but also sort of true in the overall narrative. Cody doesn't contribute nearly as much to the plot as Allirea does, and his personality is less well-defined than hers.
In Luminosity, Allirea's state of rest - what happens when she's not actively exerting effort - is to be unnoticeable. This has always been the case for her. Anything that is clearly "about" Allirea is not worth notice unless she acts to counteract that default. She's also a half-vampire. This means that her mother died an agonizing death by childbirth. And no one cared. Not even her mother.
I'm the only one that is terrified by the implication of having a LOT of part vampire AND vampire around? How come nobody even tried to think about the HUGE INCREDIBLE mess that would come out of having so many possible one sided mating?