Graendal was using a compelled slave to remove people of interest to Rand. The messenger was obvious; Rand wanted him to find her location. The merchant was also a good target, not because she knew Graendal's location, but because Rand wanted to elect a new king. She chose a slow acting poison because it would look like the merchant died from the conditions, Rand would become convinced that he killed her and all the rest. It's pretty unlikely that anyone would have noticed if they were not already familiar with the poison anyway.

You then claim that Kerb was a loose end because he knew about her hideout. You seem to be forgetting that her pets are essentially brain dead under her compulsion, so it was unlikely he could say anything even if the compulsion was removed. It was still possible, but a better plan for someone with Rand's resources would be to send a messenger to every manor in the country and see which one comes back compelled or disappears. It's a damn sight easier than brain surgery.