Yeah... just got to that part yesterday and it infuriated me too. It's not just the conversations, the whole prison area is terribly designed and needs attention as there're also the weird detections and the Warden being script-linked to the scrying eye on the level above. Trying to do the prison as one normally would is simply impossible and just leads to a buggy disaster, so I had to resort to some gameplay tricks to have it play out as it should.

The way I did it was;
  • Entered turn-based mode and then talked to the gnomes and tieflings (turn-based freezes NPCs so the conversation cannot be interrupted)
  • Killed the patrolling scrying eyes with a thunder quarterstaff while stealthed (less patrols = more time to talk)
  • Afterwards told the Warden to check on the gnomes (she leaves her post for a while and talks to the prisoners)
  • Snuck up to the evidence floor and tossed the scrying eye out the window into the chasm so I can grab Wulbren's hammer (if you just steal it the eye magically knows it was you)
  • Opened the empty cell #3 so I can go inside it and give Wulbren a weapon properly because talking to him from within the cell does not interrupt conversations (if you give him a weapon from the front of the cell, guards will attack because he talks too long)
  • Escaped without ever fighting the Warden, there's even some awesome dialogue from her too afterwards