from what I understood, the previous Lavi is bookman's son and our lavi's predecessor. He also is the one who fused with Allen's soul. The current Lavi who just became bookman, we still don't know his real, i don't think it really matters cause he took on the name Lavi as a pseudonym. It was also implied that he has some  "spiritual" link with the previous Lavi (I don't really know what, he saw him many times in his dreams or smth like that). 

And for Rhode I don't really know, just a theory but since she's been helping Nea and knows some kind of secret, she might have been colluding with Cross Marian since the beginning. Maybe since Nea got killed and transferred his soul. But that's just a theory :))