Corrected entry: It's established the ghosts can pass through walls, doors, etc. but Hetty's husband's ghost was trapped in a vault for 130 years until the door was finally opened.
Corrected entry: Hetty loses her British accent on occasion and sounds slightly Midwestern.
Correction: Hetty isn't British and never has a British accent.
Hetty is of proper English upbringing as a late baroness with a very proper slight British 'lilt' since the show's inception. That wouldn't change in the precipice unless she was narratively merging to a more midwestern-y dialect around her ghost friends in time. Being Isaac and Thor's lilts have not changed a bit.
Correction: That's not a plot hole, it's literally the plot. They specifically mention the material The Vault is made from is impenetrable to ghosts. They even say, "I love it when the mythology gets expanded."
Bishop73