In the argument over Andrew Miller's challenge of a Miller subpoena yesterday, Michael Dreeben seemed to be making the first argument about limits to how much Matt Whitaker can tamper in the Mueller investigation. 

On top of all the potential legal problems with picking Matt Whitaker to replace Jeff Sessions, there may be legal problems that John Kelly is the one who did so.

Trump is still trying to decide whether an open book test of questions pertaining to the Russian investigation is too hard for him or not. Before he decides, he's may consult with both Matt Whitaker and Vladimir Putin.

Both the author of the statute governing Mueller's appointment, Neal Katyal, and the people who will oversee DOJ in a few months have real questions about the legality of Whitaker's appointment. That could mean every single action vested exclusively in the Attorney General (including those pertaining to Mueller) can be challenged in court.  

Under a 2002 OLC memo interpreting the PATRIOT Act, Acting Attorney General Matt Whitaker can share information about Mueller's grand jury with the President.