The Forsaken who went face-on against a ta'veren have all fallen. The only ones who have survived a while were those who approached the ta'veren without being immediate threats, like Ishamael (who died when he attacked Rand full-on) and Lanfear (who got sent to the Finns when she attacked Rand full-on), and Asmodean (who died when his usefulness was fulfilled and could have become a threat if Rand involved him with his male channelers ideas). Even the Forsaken who disobeyed and sent an army of 100,000 Shadowspawn against Rand lost the Shadow the whole army.

This tactic is useless, and Moridin is well aware of this: it's his bloody mêlée reference in the Sha'rah scene, when pawns try to attack the fisher directly instead of manipulating him. That's why he has not ordered the Forsaken to kill Perrin and Mat, he has ordered them to order their followers to kill Perrin and Mat.

If Demandred leads directly an army against Perrin, he will lose. The problem is that Jordan is well aware the ta'veren twists he'd need to pull off to make that happen would be so incredible and over the top he'd lose the reader's good faith. So he wouldn't send Demandred against Perrin and a few WO, AS, two Asha'man and longbowmen.

He'll wait to use Demandred against a ta'veren who has an army to face him and it comes down to military genius vs. military genius. And if Demandred is to fall in that battle, then most likely Rand will be on the other side, even if it's Mat who leads the battle for him. That could be the big battle in Caemlyn expected at the end of TOM, though it could be anticlimatic to remove the Shadow's biggest military threat right at the beginning of the LB. Unless Demandred retreats, hald victorious yet a bit humiliated and now hating Mat with as much passion as he hates LTT...

The "hidden threat" is not so much a mystery, I think. Graendal commented in TGS that Cyndane and Moghedien ran errands for Moridin, mobilizing the Shadow, but had also been spotted by her spies (DFs in her network, most likely) in the south trying to carry out Moridin's orders to kill Mat and Perrin - so they're the hidden threats. So the "mystery" is really to puzzle out which of Cyndane and Moghedien is the one involved with which ta'veren. Moghedien has been tied to Caemlyn a lot (many of her pawns are now Elayne's prisoners) and is the one tied to Elayne, and she's been involved in Mat's storyline before (in Ebou Dar) and with people with him like Thom and Juilin, and Lanfear has ties to the wolves (she's Moonhunter) and is a far more likely suspect than Moghedien to have been the one manipulating Masema's dreams in order to get him to kill Perrin. If Jordan goes with the more obvious, it should be a Cyndane-Slayer-WC cocktail for Perrin. Mat and Elayne, Birgitte etc. will be stuck with Moghedien. The less obvious would be the reverse: Moghedien for Perrin, and Cyndane for Mat. Cyndane is more polyvalent. She has thematic ties to Elayne via Rand (Ilyena etc.), and with the Finns as their prisoner, and finally she's been involved with Mat too. But nothing has been set up between Moghedien and Perrin at all, so I'm leaning toward Cyndane as the natural choice for Perrin's hidden threat. She'll probably won't get involved directly, she'll keep using TAR, while Moghedien will more likely be using "agents"/DFs against Mat and Elayne (the Slayer equivalent for Mat is likely to be a return of Mr. Gholam, it would also be too funny if it took out Moghedien ).