Mainly because I can't see how anyone could predict that Rand would just balefire the entire Palace. Therefore there was no way for her to set up Rand with the fake Compulsion, not to mention that it should be almost impossible for her to remove the Compulsion at a distance.

I disagree about Kerb, no one would have suspected him if Nynaeve hadn't Delved the merchant. She would have died, what appeared to be, a natural death in a prison, just as the messenger had before her. Without, likely Rand's ta'veren twisting, no one would ever have found out about Kerb, who was apparently positioned before Rand ever came to Bandar Ebon.

As for your conclusion, I think you be right on, the Shadow just wasn't worried about when it would happen. Sooner or later all of the pressures on Rand would have to overload him and have him do as he almost did.