We are still assuming that the Circle, or some members within it, were responsible for summoning the Outsiders for the Red Court during Proven Guilty, and various other occasions. Someone taught the Striga how to summon He Who Walks Behind in Blood Rites. And Cowl himself, the only member of the Circle who identifies himself as such in White Night has been very, very strongly connected to Outsiders, both through the Athame/Nemesis infection, and the events of White Night, wherein we are told by Lash that Vittorio is possessed by an Outsider, who is an underling of Cowl's.
I don't think we have to assume -- as you say, the evidence is piled up pretty high at this point.
In addition, we know for a fact that Cowl was in league with the Red Court, enough to get an invitation to Bianca's ascension and conspire with them to give Lea the Athame. So that puts him in cahoots with the Red Court and by extension with Arianna.
As for HWWB, we know that Lord Raith had Outsider-powered immunity to magic. He was the one who was behind the Striga plot. It seems pretty obvious to me that
he is the reason HWWB is still kicking around after Harry killed/banished/incinerated him.
I don't see coincidence here. At the party were one guy who we know for a fact was in league with a major Outsider and a vampire who is only a degree or two of on-screen separation from the wizard we know was in league with the Red Court and summoning Outsiders.
I would not be surprised at all to learn that Cowl was either at or under discussion at that dinner party.