Sullivan cannot be a prosecutor nor can he appoint one. That much is clear in the appeal for a writ of mandamus.
Sullivan has a gun to his head. It’s the only explanation that fits his behavior.
If I understand correctly, Sullivan has already been given the case. He could make a splash by issuing a decision. His justification, even if he’s overturned, would be the deal between Flynn and Mueller.
And a splash is all the media and those susceptible to being sued would want.