There's plausible deniability, and then there's whatever this is.
Rupert Cadell fails to voice his suspicions in a timely way. There are a couple of consequences.
Brandon and Phillip get away with murder. It's the perfect crime, nearly.
In which Rupert Cadell fails to make himself understood.
Janet is there for the trial, and afterward.
Brandon and Phillip have an arrangement. It isn't a game; it's closer in spirit to a contract.