If anything, I've been more convinced that Narishma is tied to Callandor, and will take this weapon up and continue some mission that Rand won't be able to accomplish. He's already held Callandor and of all the Ashaman, we know that Rand trusted him, Damer Flinn and Eben Hopwill (now deceased) to do what he told them to do.

Min thinks Callandor is important, but really hasn't been able to come up with a significant reasoning as to what it can offer to the act of locking the Dark One outside the Pattern. I think this may be because it won't be used directly, but will be important to his allies. After all, if the Power is to be used, considering we're dealing with working on the forces driving reality, it may very well be that more Power isn't needed at all, and that what is really needed would be increased accuracy and the right combination of Talents for this. It may very well be that two or three teams will be needed to make an anchor for the Dark One in different parts of reality - a team lead by Egwene in T'A'R and a team lead by Rand at Shayol Ghul; it may also be likely that Rand will be defeated before he can finish his mission, and someone (Narishma or Logain) will have to continue with what he started.

I think some of the ambiguity lies in the fact that Logain was also destined for greatness, while Narishma is also marked for something. Perhaps he will defend the Aiel who resort to the Way of the Leaf before Tarmon Gaidon starts in earnest?

I don't think Cyndane/Lanfear will turn towards the Light. I do think that it is possible she will be instrumental in defeating the Dark One because of the way she was punished, but I think she is completely focused on revenge against all parties.