I've managed to carefully ask the questions to get the answers without killing the captives. However, as soon as I spoke to the Queen Nurese & Drago, the game disabled other interrogations and sent me straight to the stones, marking everything as "completed". Why I couldn't ask and talk with Frederique or the Princess Visanne?
Is this normal or a bug? I did find how to open the door and what the king was planing there though...