Archer suggests recording the hidden crypt and sending it to Nessie and/or Mhairi (I recall they have Watches) as well as photos of any suspicious things they find. In case something happens the proof is somewhere safe.

While he's against murder, he sees nothing wrong with occupying the room behind the Door and the Crypt and sending for Nessie.

edit - using Cwyld infection as a threat to Witches inside is a valid interrogation tactic.