I felt the same frustration. Seriously, if so many threads could be tied up in this book, why not something like this, which can be dealt with in one sentence?

I think this is a case of fewer women holding a shield made by a full circle.
For example, Rand cannot be shielded by six women when he's channeling. 13 women can shield him, but six are enough to hold that shield.
Similarly, I think it is impossible for two or even three women to shield Semirhage. Certainly, one Aes Sedai cannot shield her. However, one can hold a shield made by many more. That is what I think Cadsuane was referring to.

The Forsaken include Moghedien as well, and Nynaeve is much stronger than her.
I simply cannot understand this determined effort to make Nynaeve a weak channeler. All we need is RJ's statement that Nyn is average female Forsaken strength. That might out one or two of the Forsaken women above her, but that is all.
And if she though Semirhage was stronger than her, she would have said so. See her reaction to Alivia.

Well, she has never felt Nynaeve or Sharina close to their full potentials, so it is unsurprising she was awed when she met Mesaana.
The fact that Cadsuane and co. show no such surprise at Semirhage's strength indicates to me that they don't care because one of their own is as strong or stronger.