Well if you believe that, then shielding can never be used as a marker of strength because your suggestion places skill in such a high regard that it can make the shielder magnitudes stronger.

Which I think is wrong. Nynaeve can't believe that Berowin can hold her stating that a woman of that power cannot hold her alone. And we know that Berowin has a talent as surely as Androl has one for gateways. Nynaeve's statement proves that skill doesn't increase shielding ability all that much, especially because she makes no discrimination between the Kin and Aes Sedai holding her. Not only that, but she's never mentioned the relevance of the group that's holding her e.g the Seafolk. Shielding is shielding.

Two women can probably hold a woman of any strength... They held Nynaeve. The third woman for Semirhage is insurance, as is the case for two holding Egwene who is a flight risk. We know that Egwene can come close to breaking a strong Wise One's shield, so obviously the Aes Sedai added an extra one.