The Obstruction of Justice part of the multiple investigations can be wrapped up without donjohn's testimony. Apparently. Lots of corroboration for it from other sources that don't lie to the FBI as everyone knows donjohn will. Mueller knows the truth. There is no lie donjohn can tell that he can get away with. So what's the point of an interview? Save time. Save money. Cut to the Chase. Get on with it. Present the Obstruction of Justice part and then move to the money-laundering and collusion parts. One at a time.
