Katerine never once said only one was necessary. She just complained about having to be part of it. Silviana's comment is crystal clear that she thinks Katerine is simply complaining. The first words out of her mouth are "Because I said so" ... something a mother would say to a child who was being a brat. "Why do I have to make my bed" ... "Because I said so"



Of course they did, that's the whole point of giving it to her in the first place! The Shield being held all night is Silviana's way of saying "I'm watching you because I think you are tricky".



She said if she acted fast enough she may have a chance to win, that is not a comment about how much saidar she could hold, it's a comment on she may have a chance if she caught them by surprise.

KoD
Prologue

Abruptly it came to her that she was not shielded. Muddled she might be, but that made no sense at all. They could feel her strength, as she could theirs, and while none was weak, she thought she could overcome all five if she were quick enough.

A few seconds later they start discussing all the Forkroot in her system and wonder if she'll even be able to walk on her own. It's several more minutes before they even bother shielding her .. so of course they know exactly what is necessary. It's heavily implied in this scene that AS have been experimenting with the effects of Forkroot since Pritalle claims to have given Egwene the exact dose necessary. Clearly they are aware of when Egwene will need either more sisters to hold the shield, or more Forkroot.



You keep trotting out this quote like it supports your thought. All it tells anyone is that Egwene is suffering from Forkroot still and would have no chance of breaking Silviana's shield in that moment. Egwene has realized by now that Channeling to any effect at that point is mostly beyond her. Not to mention we spend the rest of the next two books hearing about how the ability comes back gradually. Obviously Egwene is nowhere close to her normal strength in this scene. Just like when the Seanchan attack and she has to link with Nicola and others ... that was hours after her last dose and she could still barely Channel at all.