@Crake @Duesal
We have the magic to
Soul Transfer making it quite trivial to put it into a gem and use
Speak with Soul.
As shown below he can be interrogated like any living witness and it can't hurt our cause for them to have the familiarity of a humanoid form coupled with the magical strangeness of its immaterial nature.
It also saves us from having to deal with all the tertiary concerns of "no this isn't his real body, yes we made it, no that's not a concern because..."
Edit:
Though if he chooses to be difficult we might have to use the Ethereal(?) metamgaic to get truth spells on him.
@Goldfish that approach would work correct?