Perhaps I’m missing something in the discussion of this issue, but to my mind the question of whether a president can be indicted is simple–the President can only be indicted by a federal prosecutor if he consents to the indictment.
I would hope that we can all agree that a state cannot indict the President. This would raise impossible structural problems similar to those identified by M’Culloch v. Maryland (in other words, one state would be able to control the Executive Branch and oust the President by throwing him in jail).
What about the feds? I’m hard-pressed to see why the President cannot consent to being indicted. He could allow the indictment from a US Attorney (through a grand jury) to proceed and could refuse to raise his unique office as a defense to the charge.
Why would a President want to do this? Perhaps to clear his name in a trial. Perhaps because he might think allowing a trial to go forward would forestall an impeachment. Are these likely scenarios? No. I would say a President would be crazy to let himself be indicted.