As I am taught, Jesus’ self had the duty to develop Jesus’ body to be able to recieve The Word (Cosmic Self, Christ) and that did happen at his baptism in the Jordan, by John the Baptist. " He saw the heavens opening, and the Spirit like a dove descending upon Him".
What is here important, that after this, Jesus was not a man who had a relation with this Cosmic Self, as all the profhets and gurus before, but The Cosmic Self Himself, incarnated, The Word incarnated, God incarnated. This is a central thought of christianity, but one not really understood, but seen as a mistery.
This physical presence was ended by Jesus’ death on the cross. That death was the death of the Jesus’ body.
Jesus was prepared by the esseness, a closed community of people dedicated to spiritual life.
What did happen to the former Jesus self … that’s a good question. My studies are not that far yet, and all this is what I have read, not result of direct spiritual insight.
I do not know the book you talk about but I have read things about the lost years. I would not exclude anything, but I would bet on that the hystorical Jesus’ body died on the cross, bearing the Christ. This is The Mistery of Golgotha. I cannot comprehend it yet, not through imagination, intuition, inspiration, not through ordinary reason … but this is why it is a mistery. I think this is revealed by spiritual insight only to people with very high level of initiation.