My theory it's that I think Allen don't remember his old self and his promesse with Nea because of Apocryphos and his power, like when Kanda was touch by his power and that memories was effaced and recreated.
And we know in the scans Cross said:
"When we first met he wasn't called "Allen" Should've realized that mistake sooner". (chap 203 page 16).
And you know in the novel Allen called himself Red, why Red?
She don't remember the past or that name.
Maybe Apocryphos has transplanted the innocence in the arm of Allen and he recreated his memory.
( When Neah is awakened and he saw his face in the mirror he's shocked to see the innocence in the arm of Allen,and you know that the old - Allen was friend with Neah, Neah must to know that the old Allen don't have a innocence)
But there are something I can't understand if the earl was separated for the first time in two.
There's never was 14th at first so why the memories of Noah reacts to 14th?
I think all we know about D gray man it's WRONG, maybe the "Heart" don't save anything but destroy everything and Exorcists don't save anything BUT the noah save everything
( ALSO what is the " D " in the title of the manga?! we know that the last name of mana and neah is D.CAMPBELL but the title is " D GRAY MAN " maybe Allen is a
" D "