Go back to what I said and read it again. I said I wouldn't let him in if he didn't
TRY to avoid screwing up the story. As in, I would not accept him as a player if he blatantly ignores everything we've done so far. For example, we have agreed that the final boss is the Void; I will not accept Gl1tch's posts as canon if he, let's say, make Ferox the bad guy that wants to dominate the world. However, I WILL accept him if he at least attempts to not completely throw the story off course. Most of the time I choose my words carefully when I post, so I might not have meant what you think I meant.
I know this is not my RPG. But if Gl1tch ignores what we've done before and does something that directly contradicts our already-established lore, I will not regard his posts as canon. However, I seriously doubt that we'll have to come to that, because I think Gl1tch is adaptable enough to not completely destroy our plot.
Also, no he would not recognize Exriel's creation. Xanriel has no memories of anything Exriel did.
Then why did Xanriel apologize to Sirenes way back then? He apologized to her because he had been a jerk; that clearly indicates he remembers what Exriel did. Also, there was this time period when Xanriel wasn't quite himself, but he was not completely Exriel either; that was before Exriel officially took over.