Thallium sounds like an exotic (and possibly radioactive) poison but it used to be commonly available as rat poison. The tasteless and water soluble salts of thallium were so frequently used to murder elderly relatives that it was called 'inheritance powder'.
In the game Mouse Trap, you are supposed to put the trap pieces on the board AS you play. Not have it already set up as Dean does at the beginning of the episode.
The number on the house that Mary and Dean run into is 216, which is 6×6×6.