Hmm. That's possible, but I'm not sure that this would be Demandred'd opening move. RJ's the one who kept Demandred back throughout the entire series, it seems strange that this would be his first move under Moridin's command. That's why I find it more likely to be Aran'gar. Like Slayer, Aran'gar is more directly involved as one of Moridin's tools, sent to strike where he wishes.

Of course, I agree with your thoughts about his involvement with Graendal. That's why I wondered about the timeline. If the attack on Perrin was intended to come first, then Aran'gar might flee to Graendal after her failure.

On the other hand, given that Perrin visits Rand atop Dragonmount before that battle, and Rand only visits Dragonmount after Natrin's Barrow, that hardly seems feasible. So I guess you must be right.