I'm pretty sure there are a few levels above Nynaeve, but the relevance of it remains dubious. If Alivia is 5 points stronger than Nynaeve, and Reanne only has a strength of 15 then she will consider Alivia to have a third of her whole strength as a greater potential than Nynaeve. This will obviously look enormous to her, and she will describe Alivia as 'considerably stronger'. If Graendal measures this difference it will constitute about 5% of her own strength and she'll consider Nynaeve to be very close to her. It's all a matter of perspective.

You only need to have Nynaeve 90, Alivia 93, Cyndane 96, Lanfear 100 to see that it completely fulfills every quote in the series, and still gives Lanfear only 10% greater strength than Nynaeve, which could be akin to the difference between Rand and Rahvin, and no one believes that Rand would easily defeat Rahvin.

As I said, you can't offer real examples of a pecking order among the Forsaken, except for the sole example of Moghedien's genuine weakness and her avoidance of the other Chosen. They do not defer to each other's power and never have. Lanfear is as strong as possible, and has no lack of skill, and I've never seen one of the Chosen defer to her. Quite the contrary, and she's not obviously going to defeat the others if they stand up to her.

I remain of the opinion that any of the Chosen (except Moghedien) can openly and directly challenge the others and stand an excellent chance of winning, as long as they fight intelligently. True, I wouldn't easily bet on Mesaana defeating Demandred in an open duel, but I think he'd have to fight his ass off and would be hard-pressed to win.

You continuously ignore the one true fact about the Chosen... if any of them was truly weak they'd be dead. Its actively encouraged in the Shadow. Demandred wouldn't ally with two obviously weak allies. He obviously recognizes that their strength approaches his and he's keeping them at bay until he can go for them invidually. Mesaana spoke about overwhelming Semirhage in the most recent book. It's obvious that twelve of the thirteen Forsaken are so close in power and skill that they won't easily go for one another unless they really have a large advantage. Even they know that they could lose. If Ishamael or Lanfear was significantly more powerful than any of those twelve, they would have destroyed them. Clearly they can't. You speak about Semirhage hating Lanfear but not being able to kill her, but then why hasn't Lanfear gone for Semirhage? Lanfear knows it will be a huge duel.

I think Brandon's answers have clearly shown that Nynaeve is very close to the top tier in strength. That makes her close to Rand and the other Forsaken. With training she could challenge any of them. Moghedien said as much when she suggested Nynaeve should meet Rahvin unblocked one day.