Remus Lupin

Well, you could ask him about artefacts or objects that Voldemort prizes, or places he holds in high regard. That's how Draco and Hermione were trying to trace the locations or types of horcruxes. We can find out about that without getting into the why.

I suppose it might also be useful to find out if he knows where the inferi factory is.

We could ask him about who was in favour and who was out, but his information would be old on that sort of thing. Though he might know who else was less than happy with the state of the court - no one who'd started to doubt Voldemort six months ago is going to feel more disposed toward the bastard now than they were then.