It is possible that the interpreter is part of the Queen’s retinue, rather than Fazzur’s?
Interpreter? The two people that the queen is talking to are her guests, and they are emissaries from Fazzur. Note that the troll guards are never gone. They are always there, protecting the queen. Also it’s not like they haven’t heard even more important matters discussed – they simply are ordered not to care about these things, so they don’t bother listening.
Is a different throne room to that on p. 250 of The Guide?
Yup, it’s a different one. I assume there are probably dozens, if not hundreds “throne rooms” in the sprawling complex that is the Sacred City, so the queen can probably hold court in whichever one she decides to use.