Maybe I am getting bogged down on a detail or just completly on the wrong track, but this seems like a logic error to me:
We can tell for certain that it must have been a human person amongst the following 6:
Howard, Charlie, B
ill, Lucy, Alicia and me myself.
We can black out Howard and Charlie, since they were unmistakably eliminated as options.
Now Lucy' s death is a certainty, too.
Repeat in red:
Dead people cannot act on their own. Unless they are the witch, of course

Just to be sure:
Alicia is as dead as a doornailLeaving only one option I could imagine:
Before leaving Bill set up a mechanism that pushed the key into the basement either after some time has passed or triggered through an action of me myself I suppose that blue truth is incomplete...
Sorry, if I am annoying you a little

Edit: Just saw that Alicia definitely got killed before the key got into the basement, making my second truth unnecessary.