So we have a fuller story now, and it's been made clear what we're supposed to do in order to break the curse. We must bring about the end of Reginald Carstairs. I hesitate to say "kill him" because he clearly is not alive, but for those who have objections to these methods, there is still breakfast in the kitchen. Help yourselves.
I'm curious what methods people have tried and the results of your encounters. If Clue is about a process of elimination, we need to put all the cards on the table and figure out what's missing. The house provided us with weapons for a reason, a few of them seem to be the ones described in the events outlined by the departed Mrs. Carstairs III. The diary entries provide us methods that did not work. Lyra, Kyoko, and myself attempted to encounter him in the basement this morning, intending to use the method of the pipe from the passage dated 22 MAY 1896 (please see Mr. Rogers' post for further details, if you have not already.) And while he did not appear for us to test the theory, I suspect that if it didn't work before, it wasn't going to work this time either.