ACoS, ch.40:
"It could not tell one flow from another", but it could distinguish saidin from saidar by the smell. Saidin smelled like the sharp edge of a knife, the point of a thorn. Saidar smelled soft, but like something that would grow harder the harder it was pressed. No other Myrddraal could smell that difference. Shaidar Haran was like no other Myrddraal."

However, I think it's clear that it was a Forsaken, since you need the Forsaken mark to force Shadowspawn. Demandred and Cyndane want to kill Rand personally, while Moridin doesn't want that Rand is harmed. That rules them out. Aran'gar is ruled out by her own thoughts. I seriously doubt that Moghedien would risk it, since she is mindtrapped and Moridin certainly grilled her. Similarly another Mesaana betrayal seems very unlikely, since she was just punished by SH for disobedience; besides she is concentrated on the WT.

That leaves Semirhage and Graendal as possible culprits. IMO, it is more likely that Graendal posed as her old buddy Sammael to kill/weaken Rand and play mind-games with the other Forsaken.