Shaidar Haran didn't need anyone to tell him there was a ward on the box. He's a Myrrdraal. Remember how Moiraine used to warn that most of her wards but the one meant to forewarn about Shadowspawn would attract the Fades. For the same reason, Shaidar Haran didn't need anyone to tell him where to look for what Cadsuane had hidden.

Not that Sorilea can't be a darkfriend (there's nothing very obvious to suggest she is, though), but the whole "there needs to be someone who warned SH about the ward" angle doesn't hold water as evidence.

There may be something to the fact it happened when Cadsuane was meeting the WO, but this too can be totally circumstancial considering Elza could easily have been keeping watch to spot when Cadsune left the building. It also doesn't point to Sorilea in particular, and Sorilea gave Cadsuane the key advice about Rand's psychology that made her find her the right approach to be accepted at Rand's side. She also kept working with Cadsuane after she was cast out, further dismissing the possibility Sorilea wanted Cadsuane out of the way.

The plan of Cadsuane with Amys and Sorilea was hardly a bad plan. At the time Cadsuane presented it, nothing had happened yet to make think it might backfire badly. They just all failed to see how much Rand changed after the episode with Semirhage. Rand was just too far gone in his insanity/paranoia to react as they hoped he would.

The big mistake in all of this was not to understand their only slim hope to make it work was to let Nynaeve handle everything, including Tam - and that mistake is to lay at Cadsuane's feet, as she underestimated badly Nynaeve's maturity and experience as Wisdom.

Rand was too far gone for it to have had much success, what the Pattern needed was a huge shock, and Cadsuane, Sorilea and even Nynaeve were caught in Rand's ta'veren web. The whole storyline is a string of missed signs, small mistakes - all leading to the chain reaction the Wheel needed to provoke the moment of Epiphany. There's nothing there to suggest any manipulation of Cadsuane.

The whole process by which the WO are chosen, and their training, makes it rather unlikely one of them would embrace the Shadow. They would basically need to be delusional (rejecting all they see at Rhuidean) or genocidal (hating the Aiel so much they want to destroy them). Another factor is the communal role of the WO which is so strong, so dedicated to social service. The WO are trained to be selfless, Aviendha's whole storyline was about giving up what she liked to understand her role in life was to serve her people, that her Ability her sept needed. That's not exactly a crucible to form DF, who are egoistic, individualistic/concerned all the time with their interests.
It's only among one clan, the Shaido, that a culture of denial developped among its corrupted WO, and it's seriously suggested that it all originates from the clan losing gender balance, with weak and too influencable clan and sept chiefs, unable to be the necessary match to their roofmistresses and WO, which Sevanna used to gain power. The Shaido leaders tried to establish a full matriarchy, those whose role is to be concerned with the management of the septs, and matters of ethics and balance and inter-clan diplomacy, seized control of the warriors as well, and that became an explosive combo. The only Aiel DF we've seen so far was a Maiden. A Shaido Maiden, from Sevana's sept (see a pattern here?)

It would be hard for WO to exist as DF, unless a whole secret group of them emerged. They are mostly concerned with their sept, and then less in clan business (the septs are pretty far apart), and more rarely they have to interfere in inter-clan matters. The WO deal mostly with other WO and roofmistresses. It's much easier for groups of DF to form within septs or societies, and of the two, it's in septs it's most likely nest of DF might emerge (Aiel are unforgiving to DF, spreading those beliefs to fellow society brothers/sisters from other septs/clans would be extremely risky business). So far, we know of one very corrupted sept among the Aiel, which had both a corrupted power-blind leader and one DF, and it's the Jumai Shaido (I was about to say, look in that sept for more, but in truth the only living Jumai we still have is Sevanna herself. They may have had DF WO, but we don't know any Jumai Shaido WO).

The group with Amys, Bair, Sorilea, Melaine have always worked hard and with determination. It's largely thanks to their efforts the clan chiefs have not rebelled against Rand in the Arad Doman fiasco. There's no evidence one of these WO tried to worsen the gap forming between Rand and the Chiefs (which a DF would have done). Melaine is shown as a bit more refractory to changes, but the others make her come to terms. Bair has made a mistake or two, but nothing her culture can't explain (like worrying more about keeping the clans together after Rand was kidnapped, but overlooking that Colavaere taking the throne may be something to worry about). After the LOC kidnapping, they all worked hard to patch things up, again with no evidence one of them tried to use as a wedge to divide the clans. It appears if there's DF among the Aiel leaders near Rand, they're at the periphery of the core WO (the most "difficult" CC about Rand have WO who have played very small roles so far), or perhaps one of the clan chiefs (so far it's always been the chiefs grumbling at time, in Cairhien like in AD, and the WO tying to patch things up. For the WO, in the last books, I'd rather start looking among those the most deadset against the Seanchan as we know it's one of Moridin's goal to keep Tuon and Rand apart, but so far the two very angry are Aviendha and Amys! The reactions of the WO when Aviendha returns may hold be the real clues).