I think Quentin knows that Barn is from the 20th century--he might have found out the night that he discovered the secret room in the mausoleum, which happened in ep. 780. Is that one of the ones you've missed? Q certainly doesn't know that he's become an evil ghost in 1969.
And yes, Jamison's room = David's room. It's round, so I'd guess it's downstairs from the Tower Room.
Petofi makes a very bad winner, even worse than Barnabas.