I didn't go quite into this in my last post but we know Dengeir is paranoid about the Imperials, and now he's seeing them crumble. This is basically his shining moment in life. He's at the top of the world, to the point where he's personally congratulating everyone who signs up as an Alessian soldier. If one of your characters were to pose as one and ask him questions, this would be the time. Also, like you said in your post, Finnen (or anyone) could sneak inside the jail, longhouse, etc.