Sure. Here is a save file right before starting the conversation with Isbeil/Justinia.
Thanks. So I tried a bunch of things. None worked, sadly. The theory that this might have something to do with AI avoiding throwing poison at undead is more probable if her "turning the wheel" action is in fact done via a "spell". But even if this is the case, it's still a rather lame bug, I must say. I had Fane in my party in my classic playthrough, though he was not my avatar, and didn't run into this bug.
Anyway, unfortunately I couldn't manage to make this bug go away, but you can go around it by leaving a party member behind in a different area before talking to the queen and Isbeil, then when Isbeil finishes her dialog and stops progressing the scene, have that character teleport in using a pyramid. This will interrupt the scene and Isbeil won't like that, so she will initiate combat.