I'm in agreement with you on most things. I think Mesaana's placement is still unknown. I also was wondering if we have definitive proof that Talaan is stronger than Nynaeve?

I personally think the list goes like this:

Lanfear / Alivia / (unnamed character option 1)
Sharina / Talaan / Someryn / (unnamed character option 2)
Semirhage / Nynaeve
Cyndane / Mesaana option 1
Graendal / Mesaana option 2
Moghedien

Mesaana is definitely below Semirhage and Nynaeve. We just don't know how much below. The fact that Mesaana believes herself to be a fair match for Semirhage is Forsaken speak for "I'm weaker than Semirhage." Which was obviously confirmed by Brandon Sanderson. The question is how much weaker? One level or two? I don't believe Mesaana could realistically be 3 levels below Semirhage and still think she was Semirhage's equal. So in my opinion there are only two options for Mesaana. Either she ties Cyndane (option 1 above) or else she ties Graendal (option 2 above). Nothing else fits for Mesaana.

Moghedien is definitely below Graendal, but how far below I do not know. There could be just the one level separating them or possibly several levels. My personal theory is that we know Moghedien is a coward. Cowardice doesn't necessarily equate to several levels below Graendal in one power strength. Like Aginor, Moghedien could just simply lack the skill or bravery to win in a one-on-one fight and Moghedien knows it. So she chooses to run away and fight another day (kinda like Graendal says she'll do when faced against superior foes).