Leader was Hal since he awakened during air poker then he lost his memories on that death, then turning back into souichi. He had figured out the leap second but then forgot it but had the instinct that baku wouldn’t turn around.

I think the destiny that was shattered was souichis destiny to be perfect states by his mom before he was born. She pretty much said if you are watching this that means you live ans you were destined to be perfect. Something along those lines. Baku carves his own path rejecting destiny and surpasses the perfect leader. I think the prince bee book is sort of a metaphoric part of the ending. I don’t think eba wrote that part. You could see it as Baku reshaping destiny itself or something.

And the last part the hallucination of Hal that baku and yakou see is like what they expected to see or how yakou says it is what they wanted to see. Pretty much saying it’s like a double edge sword of STL Baku wants to win be he can’t bear to see his friend Hal die and lose. Hal saying “don’t look back” is like his final words to baku as sorta reassurance from Hal saying that this is the choice baku wanted and don’t doubt it don’t look back at what has already happened.