But if they are going to get that kind of access to your prisoner, a shield wouldn't have done any good anyway. I would, of course, keep channelers guarding them as I would anything I thought might be a target of interest to Shadow channelers, or else use wards of some kind on their cell.

I am talking about severing as a simple security precaution if political necessity or whatever reason precludes this. My point is basically that if they are going to hold them prisoner, hold them severed, rather than shielded. If I had been in charge in Caemlyn, I'd have given all the sisters a very fast trial and a long drop on a short rope before Pelivar & co's note requesting safe passage arrived. But Elayne anticipates a reprimand for merely stripping them of their Great Serpent rings. If I had been a person of any influence in Rand's inner-circle, I'd have Semirhage dead already - by naming herself Semirhage, she forfeits even the basic civil right to a trial. But for whatever (idiotic) reason, the characters have chosen to hold them prisoner. If you are going to do that, IMO, stilling is safer.