Egwene is under Forkroot. It doesn't wear off nearly as quickly as you claim. When she was being handed over to Silviana and back to Katerine and Barasine she was still under the effects. Nynaeve and Elayne and a number of others show us that a Channeler doesn't have access to their full power for hours and that it comes back gradually not all at once. Egwene is dosed several times a day because of this. Egwene is only just able to embrace the Source when Silviana takes over the shield, so there is no way the forkroot had left her system enough to allow her any realy strength.

The only time we see anyone consider how many Sisters they need with her is here:

They glanced at each other. Of course they'd been listening. "You'll want to go determine for yourselves what has happened," Egwene said. "Why haven't you?"

The two glanced at her with annoyance. "The shield," Barasine said. "We've been instructed to always have at least two to maintain it."

"Oh, for the. . . ." Egwene took a deep breath. "If I vow not to embrace the Power until I am properly back in the custody of another Red sister, will that be enough for you?"

The two regarded her with suspicion.

"I suspected as much," Egwene said. She turned to a group of novices who were standing in a side corridor, pretending to scrub the tiles on the side wall while they gawked at Egwene.

"You," Egwene said, pointing to one of them. "Marsial, isn't it?"

"Yes, Mother," the girl squeaked.

"Go and fetch us some forkroot tea. Katerine should have some at the study of the Mistress of Novices. It's not far. Tell her that Barasine requested it for use on me; bring it to my quarters."

The novice scrambled off to do as asked.

"I'll dose myself with that, and then at least one of you can go," Egwene said. "Your Ajah is collapsing. They're going to need all of the clear minds they can get; maybe you can convince your sisters that it is unwise to let Elaida execute Silviana."

The two Reds glanced at each other uncertainly. Then the spindly one whose name Egwene didn't know cursed softly and hurried away with a flurry of rustling skirts. Barasine called after her, but the woman didn't return.

I don't know how you can ignore how many forkroot comments there are throughout this part of the book ... we know Egwene is dosed Morning, Afternoon and Evening to keep her from having enough power to be effective. And we know that the directive when she isn't on Forkroot is to have at 2 Sisters shield her at all times.