I know this is old, but this is what I thought when I read the manga. Sometime when people translate it's hard to really read the actual meaning, and as I said, every post is their opinion.
Mana was indeed a Noah, and was also a fake father of Allen. The person in Allen was suppose to be the next king or something, but he ran away. The Earl knows a lot about Mana and the other boy of Allen. Allen is supposedly be the 14th Noah. His real family is a mystery and most likely Allen is the humanoid form of the 14th. As I watch or read the manga/anime, it shows that Allen only remembers the night of his abandonment and the kill of Mana; anything beyond who he was or where he came from was a mystery. I think that the 14th is Allen and that he sealed his memory.