By chance Nynaeve decided to further investigate the case concerning the dead messenger & Lady Chadmar on her own (contrary to Nynaeve, "Rand had determined that his hunt for the Domani king had hit a wall with the death of the messenger" ). Later, just before Nynaeve leaves the dungeon, she spontaneously decides to Delve Lady Chadmar to see, if she is ill and identifies the poison. This leads to Kerb, and Nynaeve manages to unravel the Compulsion on him (which is quite difficult). By chance or ta'veren, Kerb didn't lose his whole mind in the porcess, but does manage to utter at least the location of Graendal's palace, when he is asked by Rand.

While there certainly was some luck on Rand's side, this scene isn't that illogical. On the other hand, it's pretty crazy to think that Graendal could foresee all this, especially relying on chance, to lure Rand to her. If Graendal made a mistake, one could - in hindsight - say, it was apparently ordering the death of Lady Chadmar by this slow-working natural poison, which could be detected by a capable Wisdom AS in time (though as said, the Forsaken often make such mistakes, and it could have worked).