I can certainly confrim that you will get Speak with Dead even if you pass only one wisdom check ...
I have the feeling that you will get it even if you fail even first wisdom check, but im not exactly sure about that one right now. :-/
Therefore i dare to presume that its only first of possible rewards ... that is not yet implemented.
Either that, or you need to have (again not yet implemented) Necromancer specialization to get its full potential.
I still dont understand why cant we change Race for our hirelings.

Lets us play Githyanki as racist as they trully are!