Reanimating this thread because I just finished Affinity too, and I had similar questions. Yes I think Miss Prior did drown herself and Vigers was Ruth, but -- as the OP asked, Who was Peter Quick, was he a fraud? What was his relationship with Mrs Brink? Was Mrs Brink a fraud or a victim? Amazing book though. And I didn't see the twist coming.
I see this is a zombie thread but as Peggy doesn't get it either...
yes Miss Prior drowned herself
Ruth Vigers was Peter Quick. She and Selina Dawes put on an act together to fool people that he was a real person and speaking to those in the audience. Mrs Brink was a victim. She took Selina in because she believed in her "powers" and believed her mother was speaking to her via Selina. She then died of a heart attack I think, brought on by the shock when she found Selina and "Peter Quick" physically assaulting the young girl.