First, let me put the timeline.
- Was born 90 years ago. From the current timeline.
- 52 years ago, they entered Grandline.
- Brook expired in a known timeline after that.
- It allows the user to come back to life just once.
- Once the user dies, the soul is returned to the living world.
- Brook became an exception. By the time he got back to the real world, his body had decomposed, and only a skeleton was left.
- Now, what I speculate is that the fruit is reincarnated again after the user dies, but Brook is never gonna die. Unless his entire Skeleton is destroyed or a soul ability like Big Mom interferes.
- Brook created an exception for himself.
- God Valley happened 38 years ago.
- And current timeline is around 55~75 years ago.
- My theory is Xebec ate the fruit after Brook expired.
- This explains his dialogue of " I'll come back ".
- Like Brook, his body was entirely destroyed by Marines, and he had nowhere to go.
- He chose his 1-year-old Blackbeard body.
- Because no way a child knew this much info and where to wait.
- Xebec went back to Whitebeard, his own First mate. Stayed underground the entire time. Away from the Marines' eyes.
- Saw the fruit and took the advantage, Yami Yami was eaten by Blackbeard.
- And Gura Gura was eaten by Xebec's soul which reincarnated.