She does it at every chance, she carefully challenged Galina in LoC, Silviana here and later she tries it with Saerin. It's part of her nature.

As for the Forkroot, we've all seen it take hours to wear off. In TGS, Egwene takes a large dose and cannot channel at all. Then she meets with Verin, and spends the better part of the night reading Verin's notes. After hours she can still barely channel a trickle. Nynaeve and Elayne had the same problem the first time we were introduced to Forkroot in tFoH

By the time they reached the kitchen, Nynaeve was tottering about without help, poking into the cupboard full of colorful canisters. Elayne needed one of the chairs. The blue canister sat on the table, and a full green teapot, but she tried not to look at them. She still could not channel. She could embrace saidar, yet it slipped away as soon as she did. At least she was confident now that the Power would return to her. The alternative was too horrible to contemplate, and she had not let herself until this moment.

And we also know that Egwene was fed Forkroot at regular intervals so Katerine likely thought 2 guards a waste of time.

As to Silviana calling Egwene tricky, I think that's simply a nod to Egwene being smart. They did just see her turn the Harbor Chain into Cuendillar, and then there is Leanne's reappearance. Who knows maybe Egwene and company have figured out a way around Forkroot or perhaps Silviana was afraid of a rescue attempt... rumors of Traveling have been flying around for months!