True, but what I'm think of is somewhat more pragmatic. It wasn't to hide compulsion on just one person, but anyone else there whom she may have compelled, directed, or used. There were probably several people in the Bandar Eban whom Graendal used - it is the capital city, after all. It's highly likely that she had her fingers all over the place manipulating and compelling. I imagine that Kerb helped in several places (but I admit, there isn't very much evidence for that).

However, there is evidence that Kerb has been in Bandar Eban for a while. Possibly months before the messenger arrived. In TGS, Rivers of Shadow we learn that the messenger arrived two months before Rand arrived. We also learn that Kerb had been around for quite a while, several months at least:

"The lad," Jorgin said. "Nobody important. An apprentice we found among the refugees a few months back. Quite a lucky find - our last apprentice ran off on us, and this one was already trained in -"

This passage tells us Kerb was in place a few months prior, before Graendal definitively found out Rand was coming to Arad Doman, and that he was already trained - to prepare food for prisoners, apparently. His origins 'among the refugees' is somewhat doubtful, since Graendal obviously had compeled him at some point before, but it is not unlikely that he spent some time among them, further lengthening the gap between Rand's arrival and Graendal's instruction. I suppose it is possible that Graendal sent Kerb for the express purpose of killing the messenger and to trap Rand, who would come later, but I find it more likely that Kerb had other purposes .