You should have realized that the Forsaken make mistakes all the time, because they are flawed and arrogant idiots, who consider themselvess godlike. This leads to their deaths, and therefore they won't win in the end. Graendal was even outsmarted by Sammael, and there are countless other examples, where the Forsaken do rather stupid things, at least for the omniscient reader.

I would bet everything I have, that RJ rather lets Rand look clever and Graendal stupid, than Rand looking stupid and Graendal clever. A twist like: "Haha, Graendal actually wasn't in the palace anymore, Rand still has to deal with her somehow", seems completely redundant, especially that late in the game.