After TGS and BS's revelations one can hardly deny that Graendal became at least less likely to be the killer. This automatically makes it more likely that it was another of the possible killers, e.g. Lanfear, Taim or Slayer.

However, the problems with Lanfear remain:

We don't know if Lanfear and Moiraine even got any wishes. It seems unlikely that neither Lanfear nor Moiraine would come up with the idea to wish for their return to Randland then, like Mat did. They apparently both became prisoners.

As you mentioned, it also doesn't make sense for Lanfear to use one wish to get the opportunity to kill Asmo, if you can wish to kill Rand, Aviendha, Egwene or a hundred other much more important things.

Furthermore, why would Lanfear wish it only several hours after she fell through the doorway ter'angreal? Mat immediately got his wishes.

We also just assume that the Eelfinn have the ter'angreal and the prophetic ability (normally the Aelfinn can only read the future of the person before them) to achieve the murder.

All in all, Lanfear seems very unlikely as the killer.