You ignored the second possibility.

That Graendal did indeed invert the weaves but Nynaeve found them since delving allows you to see inverted weaves. I dont think thats necessarily a huge leap.

It could be sorta like observing a black hole. You cannot see it directly since no light can escape, but you can know its there from the effect it has. Delving analyses flaws and problems in the body. Maybe Nynaeve was able to see it that way.

Imagine an invisible man lying on a snow. You might not be able to see the man, but you would know he is there because of the clear intendation he makes. You would know his shape aswell since you could see it in the snow. Just like Nynave might be able to "sense" the weaves based on their effect on the brain.

To be honest I am not entirely convinced this is true so I am being more of a devils advocate here. The descriptions do imply that Nynaeve saw the weaves themselves if I recall.

Still, it could be possible, and I would not find it overly problematic if BS said that they were inverted but Nynaeve found them anyway and used that to explain how Graendal was so easily found.