I unfortunately don't have the game in front of me, but last week we played the 2nd case ("The Mystified Murderess") and it turns out
Spoiler (click to reveal)
Guy Clarendon is the society thief. Looking at the dates in the case description, however, (and again I don't have the exact dates in front of me), our understanding was that the latest robbery had occurred a day or two after Clarendon's death. This lead us to conclude that Clarendon wasn't the thief.
Did we just misread/misunderstand this?
Also I'd be interested if anyone has an errata document for the game. I've seen such a list referred to on the internet, but haven't been able to find it.
The paper says the last robbery was July 1. Guy performed the last robbery the night of July 1. He was killed after he got back to the hotel. Frances came in the next morning (July 2)and that is when the body was discovered.
To anyone attempting to read this thread without seeing any spoilers: Although the OP seems to have a reasonable question, the response above indicates that you can play and solve the case as written, without any need for editing or errata.