POV and Jesse being way too difficult to replace convingly - and this is true for most sisters as well. Most importantly, Mesaana had no idea what the AH were up to and charged Alviarin to find out. She's most definitely not one of the plotters, and we know none was BA beside (Galina was kept out, she would not have gone along with stripping power from a woman close to her and under her influence to share it with the other Ajahs...). AS are perfectly able to spread division and chaos on their own, it's why Mesaana's job and Alviarin's were so easy. There were cracks everywhere, Mesaana merely had to push and pull here and there to get the AS to open them into chasms...

Norine IMO is probably to exclude, because the Whites are a close-knit isolated group (who love to debate and argue with each other, to boot), and that would terribly hard to replace one of them. These are women who've known each other for many years..

Alviarin had the gist of it right; if Mesaana has an AS identity, she must be a little known sister, perhaps away from the Tower from many years... or a very young, friendless sister.

So I still believe the most likely sister we know remains young, friendless Danelle, who very strangely got herself involved with the Tower Coup conspiracy then let herself be pushed aside to fade back into anonymity, as soon as Alviarin was in full control of Elaida.

And yeah, it's also my theory that Danelle is just Mesaana's main disguise, and that day-in and day-out she's used others, to move among factions. I think if she's portrayed other AS, she's done so only in a very limited way, briefly, and generally avoided contacts when in those diguises. I think she's also used the disguises of clerks, servants and the likes, and quite possibly novices, all depending on her spontaneous needs.

I still also stick to my theory that Mesaana has built herself a little network of "children" among the novices (which she used her young Brown alias of Danelle to gather one by one, in private lessons and such. Mesaana is an expert at corrupting young minds to the Shadow). This would explain well her comment that each day more join her side in TGS. This would explain why RJ bothered to involve Egwene so much with the novices, and this would come as a surprise that Mesaana still has a network infiltrated even with the BA gone. It would also explain why Mesaana cared so little for the BA, if she already had her own, secret spy network, which attend to many sisters and report all they overhear, and how things stand in the Ajah quarters, and who meets who and so on, and so on.

With this theory, it follows that Mesaana could replace any of her novices for an afternoon (hiding her in Danelle's library quarters in the meantime) if one of them got an interesting enough assignment to Mesaana, like attending this or that Sitter or AH.

Portraying a lonely young Brown has some advantages. Few know her well, and she has few contacts. The fact she seeks to remain away would not appear weird to anyone - she's known not to be sociable. She's not old enough to be expected to be an accomplished scholar just yet. Danelle would give her easy and discreet access to an important BA like Verin (I mean, she could watch Verin's comings and goings above the library, and know when to contact her, not that she revealed her alias to Verin). She's young, and the woman who know her best would be Sheriam, another BA, and one sent out of the Tower, with many novices and most of the advanced Accepted who could know Danelle better, as one of their teachers... Danelle also has striking blue eyes, Mesaana's only food feature. That could be what Alviarin found familiar with Mesaana's face but couldn't place. Mesaana's only little bout of vanity might be that she doesn't disguise her eyes as Danelle's are so similar to hers. AS didn't know how good Illusion can't be, so they might brush off as not noticing before that Danelle's eyes were so great. Finally, not only a young Brown like Danelle had free access to novices and Accepted, especially after so many teachers left with the rebels, but Browns have two sets of rooms, above the library and in the quarters. Such an arrangement would let Mesaana stop playing Danelle for days at need, without anyone wondering too much where she's gone. Of course, a Brown disguise would let Mesaana access the library to research pretty much anything without raising the sort of attention a Red, Blue, White, Yellow or Gray would raise (just see how weirdly Alviarin was looked at for exiting from a little visit corner about arithmetics... and yet she's a logician!). A Brown, especially young and little known could pick books on Tower history, records of customs, the DO' seals and pretty much anything without anyone finding it odd: Browns will read and research anything.. and beside Danelle may well be teaching some of these topics to novices.

As someone like Danelle, Mesaana could also participate in Tower life only as much as she wished, and no more. That, and minor disguises she adopts briefly to move around or reach places Danelle can't be seen at (that might include even replacing a SC member for the time of meeting, and things like this. We know she's been in contact with Verin - she could easily have asked her to skip a SC meeting to take her place and ascertain their full loyalty to her orders(if Verin was SC, and it seems she was).

It seems like a very practical and convenient method to me, letting Mesaana get closer to the thick of things for the Tower Coup, than step back afterward. This would also explain how Mesaana knew so much about Elaida's early meetings (Danelle was initially in the inner circle), until she had convinced Alviarin she could never be sure how much Mesaana knew and got trained to report accurately every last detail... After that, there was no point for Mesaana to remain so closely involved...

I'm not sure I see so many other sisters we know of that would give Mesaana so many advantages and that she could realistically have replaced. The other theories tend to emphasize things like "as a White like Norine, Mesaana would enjoy the debates and could withstand her own"... It seems a bit superficial to me. She's not there to feel at home or enjoy Tower life, and I doubt her chosen disguise has much to do with personal inclinations. What Alviarin little understands is that Mesaana is not interested in the trappings of power or in being seen wielding power. She mocks Alviarin's petty ambitions. I think this suggest Alviarin has chosen a disguise fairly low and discreet, away from the top sisters she let Alviarin handle. I don't think she chose someone obviously as prominent as Norine, who could have rivalled Alviarin to be chosen as Keeper, and she is definitely not an AH.