where the real sloppiness and lack of continuity was was in the family's treatment of the character...
after 1897, during the "Grant Douglas" period, he was a total stranger, the children were afraid of him, and he was associated with the ghost that had haunted the house the previous year...
then the storyline shifted to 1970 parallel time.
when the story returned to "the present" everyone acted like he was an ordinary, everyday member of the family who had always been around. Roger was absent so he sat around sipping sherry and bickering with Liz like he was her brother. when David was taking pictures during the "Gerard" haunting Quentin told him he'd been asking to borrow his camera for "a year". a year??? a year earlier Quentin was a ghost haunting the place. no one was borrowing his camera.
they just glued the sideburns on, dumped him in his old Victorian room with that stupid gramophone ( to keep the "brand" consistent ) , and hoped no one would question the lapse in continuity.