I'm confused. What makes Baku so sure he will be alive after losing the StL.
If Baku didn't know that Hal is the leader then there is no guarantee the leader will spare Baku. If Baku know that Hal is the leader then the problem is that Hal lost his memory and his plan would fail.
Maybe at the time of StL, Baku know that the Hal have memory loss (since it seems like the current Baku know about it).
Baku could have got the location of Hal's gamble from the guy who was calling him in the previous chapter or from the Prince Bee book, and find the computer which contain Hal's memory. (Which is very unlikly since Hal know that Eba's dead so this place no longer have any use. I think Hal would probably dispose everything relating to his memory loss and leave a clue at that location to talk to his attendants about his memory.)
Or maybe Baku saw Hal just as he leave the location and find out he lose his memory and is related to Kakerou. (I'm imagining that Baku found the location of the gamble, saw Hal with his attendants and referees cleaning up Eba's place and somehow deduce Hal's memory loss and relation to Kakerou. No evidence on the likelihood of this scenario though.)
Or maybe Baku know everything from the beginning. Re-reading a bit, at Ch.321, Baku said to Hal "you can just do what you want". Maybe Baku already found the location of Eba from the Prince Bee book and found out everything, so he said that to Hal so Hal can decide what to do next. To be with Baku side or Kakerou. (Which was the question Hal was asking himself, so maybe he knows that Baku knows?) Baku was a bit suspicious on why Hal would want the book so much, he asked alot of questions at the bookstore which Hal don't have the answers. (Not sure how he avoided being kicked out/killed by Eba though)
Or maybe (again), Baku know everything from the very beginning when he was at the bookstore. I am a bit suspicious by the fact that Baku just happen to walk in at the bookstore and "found" the Prince Bee and that he wants it because he's a "collector of rare goods". Maybe he did this knowing that Hal would be the leader someday and get in his good side and would spare him in the StL so he can have a more thrilling gamble.
In any case, even if Baku know about Hal's memory loss, there's nothing Baku could do to regain his memory. (Plus considering how shocked Baku is when he saw that Hal is the leader. I'm not sure if Baku know anything at all.)
Maybe, it's all just a gamble?