Dear PhilDante
please just watch the video from the 7th min onward .....https://youtu.be/ANp00jDxV6Y
its just right after you place the 4 books on the stools......and you enter the next chapter, .........the doors should be open by DEFAULT and will only close after you entered the combat arena
so it sounds like something gone wrong here, if you where smart to have a save ..........just reload and do it again ..........yea its a massive quest in time ..........but it far better than never finishing it
PS ........and if by any chance that you have altered the time-scale ...........your stuffed .......try and remove any mods that do so or set any INI manual changes done affecting time back to there default value witch is 20