What do we know about the actual composition of the Council (before Chammady changes it)?
There are eight nobles on the Council, four of whom are patriarchs/matriarchs (Vassindio Drovenge, Eirtein Oberigo, Lord/Lady Salisfer, and Lord/Lady Dioso (Lord Lucio Dioso IMC)) and four of lesser status. (PF30 p. 11) We don't know the families of these other four, but we can possibly eliminate some, or at least speculate.
It seems most unlikely that Visbaronetess Aulamaxa is a member, and if it had been a member of her family assassinated in chapter six then I suspect the adventure path would mention it. I feel we can rule them out as belonging to the Council.
Similarly, given the nature of Calseinica Nymmis' family's involvement in chapter six it seems rather unlikely that they are represented on the Council.
Lord Ghival has systematically impoverished the rest of his family, and the way they are discussed suggests that none of them had any pull with the Council, let alone a seat on it.
Lord Mayor Arvanxi seems to be the only living member of his House in town (since his older sister Lonosete married out of it), and he himself is clearly not a Council member.
None of the Arvanxi client Houses (Mhartis, Ciucci, and Rasdovain) are given any kind of recognition by Council agents, and the kind of care to cover one's tracks when moving against fellow Council nobles is not in evidence. I feel it's safe to rule them out.
The fact that agents of the Council seem to have caused Sascar Tilernos to disappear suggests that House Tilernos is not affiliated with the Council. (I might be wrong about this, though; I'm not sure I actually understand the subplot surrounding her disappearance.)
House Chard is an interesting possibility, but I can't help but think that Paralictor Chard might have mentioned the assassination of one of his family members by the Council when talking to the PCs.
The description of House Rosala in chapter six suggests none of them are (or have been) sitting on the Council.
House Ulvauno is a possibility, but they frankly don't need any more focus.
I like House Khollarix for one of the four other Houses. They aren't expanded upon within the body of the adventure path, but the excellent fan-created document on the Dusk Market ties them to the illicit slave trade.
This still leaves lots of possibilities: Atenaar, Cemaine, Chillarth, Etrovain, Grulios, Ici, Imvius, Julistarc, Lorialn, Mezinas, Mironeth, Missepe, Nolmon, Phandros, Rufano, Rustachas, Seidraith, Starnon, Ucarlaar, Vitaron, and Xerysis. For my campaign, I think I'm going with Khollarix, Mironeth, Seidreth, and Vitaron.
Aside from the eight noble members, do we know if there were any others? I can't seem to find an actual number anywhere, though we know they all comfortable sat around a table in Room D15 of Walcourt.
Have any of you done anything in terms of nailing down membership further?
This is very well done! I was just in the process of creating this exact resource, and you beat me to it. I would add Lord Ghival as under Chammady's thumb.
The reason that I think that the Council is smaller than having a representative of almost every House is that committees that size have trouble getting anything done, and for all of Chammady's complaining the Council has managed to keep crime organized in Westcrown for centuries. Even having eight members means that their meetings will be long, but manageable. If your list works for your game though, you should of course stick with it.
Thanks again for the agent list!