The locked door should be opened after defeating some undead in the middle room. A couple people who defeated Barnabus before going through that room found that the undead where not there, so the door could not be opened. I don't recall anyone else having a problem using the dragon stone. At least one person returned to the cave some time later and didn't have any trouble (the undead were there, and the door opened when they were finished off).
The Book of the Dead that Igor wants can be found in the third room, as well as a skill book IIRC. If you picked Jonelath as your necromancer then not getting in isn't a huge loss, though one person who had Igor and got locked out said their quest log stated that they had already given the book to Igor, and that they should talk to Sassan about it.
