Was David Collins at a mental institution while he was "away?" Or perhaps spending a little time with Julia and that medallion of hers?
How quickly David's family seems to have completely forgotten the reason for the seance only a few "weeks" ago. Prior to Vicki's trip to 1795 the household was in a state of panic over young David who claimed to see all sorts of things from coffins in basements to secret rooms in mausoleums to Sarah the ghost.
Did David - and his family - simply forget about David's morbid fear of Barnabas? Have they decided that the entire Sarah episode is over and done with, and not worth concerning themselves over any longer? What of the fact that Victoria, obstensibly David's governess, has learned that there is in fact a secret room the the mausoleum? I guess Vicki is so self-absorbed and obsessed with Peter/Jeff she hasn't bothered to consider that David was telling the truth about that room all along . . . and possibly about the rest of the things he saw as well!