If he wanted to keep her death vague, he would have sent that idiot lord in and have him come back weeks later clearly under the influence of Compulsion. Rand would then follow and destroy the Palace, and we'd wonder if Graendal was inside.

Instead, Brandon had Rand follow him there closely so that when he destroyed the palace and Graendal, he would have enough power to see the Compulsion instantly unravelled, and thus confirm her demise.

Graendal's demise and the reversal of the Compulsion is as obvious as Rahvin's and the reversal of Mat, Aviendha and Asmodean's deaths.

The reason Brandon wants to keep it vague is because we'll have a Graendal PoV in the next book.