I think it makes a lot of sense for the Earl to be Mana because.....
in order to use the Innocence to its fullest potential the user has to love in order to save.
I'm sorry if this is confusing. But Dark Matter and Innocence are opposites, right? and we have already seen that Dark Matter is powered by
Hatred . Evidence below under spoiler tag.
Here Alma states that
"if you hate, your power flows to the extent of that hatred".
So if they're opposites than that means Innocence must be powered by
Love and Allen being the only exorcist to love Akuma (expect maybe Kory) makes him the closest one the innocence (like Apo-chan said in a very creepy way), right?
Which brings me to my next point, Allen saving Noahs.
I think the main reason Allen couldn't truly hurt Tyki's Noah (save tyki from his noah) when he slashed him is because he didn't love the human enough.
I think if Allen really wants to destory the noah he has to love the human and not a little but by a lot!... and if the Earl is Mana... maybe Allen will love enough to want to save him from his madness.
I'm sorry if this was confusing.