Again, Harry literally suspects that his entire idea of a "Black Council" is wrong and it might all just be Nemesis.
That said, Cowl doesn't seem nemfected. Nemesis infected agents tend to pretend they are themselves and do weird and creepy and insane things in the shadows and only reveal themselves when caught (if they ever do).
Cowl seems to have his own plan.
I would say it's likely a bit of both. The Circle (as the Black Council might call themselves) and Nemesis (and the Outsiders) are quite possibly in league - who is in charge is more interesting to note.
As for Mab and Anduriel etc. not knowing...who says they don't know? They may just choose to keep certain information to themselves. Anduriel likely hides information from Nic after all. Mab is under no obligation to reveal everything she knows to Harry or anyone else. Remember, Vadderung knows at least some of the players. He admits as much during Harry's first meeting with him in Changes.
Beyond that, it isn't like the "Black Council" doesn't have it's own heavy-weight support. It could well be that there are powerful entities hiding much of the activities and identities of the members of the Black Council. And perhaps it's also that they are just very, very clever. Human intelligence often seems to be undervalued by supernatural beings and yet time and time again we see it triumph. Cowl definitely seems to be highly intelligent. I suspect he isn't the only one on the team.
The name of the organization is The Circle. It is possible that Cowl is the entirety of the leadership of the Circle, but I don't lean in that direction... I did consider it (I think in that other thread) but it doesn't really ring true.
I mean, I agree that it probably is the Circle. But we don't know that as a fact. It's just a possibility. But a highly probable one at that.
I am not so sure they have leaders, myself. But yes, I doubt Cowl is the ultimate leader if they do have one. Their very own "Merlin" if you like. No, I suspect there is something worse in the shadows, as always.
Also, I am not convinced about the idea of cells. I would need to see more evidence of independent cells operating, and perhaps even working concurrently maybe even with competing aims. So far, everything that seems to be linked to the Black Council all seems to be done to further the same aims. Their doesn't seem to be an element of isolation, more a nebulous hand guiding everything along.
Given the way Harry uses a ton of different names for mother winter… and clearly not all of them… I think 7th son and nameless and several other titles would work as names… not as well as a proper name, but I don’t think not having a proper name is that great of an advantage.
You might want to re-read Storm Front. Lots of emphasis on the power of real names over nicknames and monikers. There is also an old WOJ about how if the White Council took on Mab, they would need all their wizards PLUS her True Name (so using Mab wouldn't be enough to actually control her).
That said, it isn't everything or nothing either. Harry bound Ethniu using that name. Is that her True Name? Maybe, but like Vadderung or Odin that name merely represents a facet of the whole being. So I doubt it. But perhaps it's enough when combined with other things (like Arma Christi and Demonreach etc). Maybe it's enough because they choose to bind themselves to a form, and by using a name and diminishing themselves into a single form, they leave themselves vulnerable to being controlled by a moniker. Perhaps that's why Vadderung and Drakul have held multiple identities over the years. Perhaps they see the value in being forgotten every few centuries and not allowing themselves to be at risk of being bound.