Holy shit guys I just had an epiphany! Hear me out, I'll give you the base rundown of my theory:
A.)Allen is the previous bookman (already obvious)
B.)The millennium earl IS the heart
C.)There are actually three millennium earls
D.)Allen is the millennium earl
First off Allen is a bookman, that's fairy obvious but how did he get his innocence? Answer: Neahs blood because the earl is the heart. Why? Well, remember what the Earls original name was? Adam, the first human, as in the HEART of humanity. Allen got some blood on his left hand and that combined with his memories of Neah and being killed caused him to reset.
And why do we know that the innocence is blood? Well besides crystal type innocence what did the police say to Allen at the beginning of the series? "My god your hand is covered in blood! ". Next is mana and Allen, how did he become an akuma when he's the millennium earl? Well remember the mirror scene? I think that the mirror scene ACTUALLY happened when mana got killed, the insanity of the ME split off and became the one we know, while mana stayed behind? And we never saw mana's soul go to heaven, where did his soul to? In Allene left eye as his curse. Therefore Allen has both halves of the millennium earl, and do you remember who the heart is? That's right the millennium earl. I rest my case.