I think the intent in Turncoat was for Peabody to become Warden (he is an expert on summoning and binding, his book on Erl indicates that) but to do that he needed the current absentee Warden Dead, and if that was Fortier it explains why Peabody aimed the Captain at him for an otherwise motiveless killing. Unfortunately Harry by pure fluke beat him to it. The Murder and Harry acquiring the wardenship happen in the same book, coincidence, perhaps, but Battle Ground showed Christos was not the stalking horse he was portrayed to be earlier destroying that weak motive, and it would depend on his election succeeding. Killing the Warden means the wardenship is up for grabs definitely unless some idiot just happens to blunder in.
I completely agree that LaFortier was very likely the previous Warden. McCoy and Harry thought the murder of LaFortier was to drive a wedge into the White Council and get one of the Black Council on the Senior Council. This honestly never made any sense at all. Why would the Black Council need a seat on the Senior Council when they had Peabody "taking care" of everything? In reality, the Black Council was very effectively controlling and influencing the Senior Council already. Frankly, this is more of Harry and McCoy thinking everything is about Wizards.
Additionally, Wizard Christos doesn't seem that bad -- just a self-important blow-hard (like all Wizards). He doesn't seem to be particularly anti-Dresden either. Moreover, he was seriously wounded -- very badly burned -- during the Battle of Chicago and was conveniently absent from the vote against Harry. Indeed, it seems likely that the only Senior Council members present were Merlin and Mai (and maybe Liberty) -- Christos, Listens to Wind and McCoy were all hospitalized. Sure, he went ape-sh*t on Harry during the Arianna incident, but let's be honest -- Harry deserved every bit of that and more.
It is also clear from Turn Coat that Peabody and Rashid knew of Ways to Demonreach. However, they are clearly different Ways. Rashid came to the Docks and did not set foot on the island, whereas Peabody came to a different part of the island. Even if a Way from Arctis Tor exists to Demonreach, and Rashid used that, Peabody's way is not the same. Maeve, Lilly and Fix used hang-gliders to get to the top of the island. Binder and the White Court vampire used jet skis.
I also believe that a Way made on Demonreach would go to Hades, it would not go to the vaults, however, but the prison part of Hades. This is why Harry couldn't open a Way on Demonreach and get to where Nicodemus wanted to go. Hades is probably quite large.
Finally, while Rashid probably has max respect from Mab, I doubt she let's anyone know her secrets.