So, if I understood well, Marian said that it's impossible for Allen to be our Allen because he is too young. For him, Allen is the host of Nea, and the only Allen at this time is the dog.
And the dog was really weird in this chap, for me, it was possessed by Nea since the incident. He keeps trying to call Mana or haunting him, saying to him that he is always besides him or to keep wandering but for nothing.
And Nea Dog kept an interest into Future Allen, maybe because of his innocence (maybe the heart as Marian suspected it), and started to approach him to possess his body.