still going with my theory, Nishina= Lucas = way too easy of a plot for this manga! beside Lucas is way to intelligent to keep pretending even though everyone is suspecting him right now.
but now we know for sure Nishina isn't Ashley, I think even more than knowing who he was, knowing why he think he is Ashley if more the question. Can a soul reincarnate into to ? but still the fact that he doesn't have any Ashley related memories is a bit off.
Other that that 2 major factor :
Brat maybe alive... maybe he isn't in the same classroom? maybe he didn't remember yet ? we don't know
the other think is... We got a story involving God himself! speaking of god's sin make him more "a piece on the chessboard" that the "player".