Two Justice Department Office of Legal Counsel opinions hold that a sitting president is constitutionally immune from indictment. Mueller, appointed under executive authority, is obliged to follow them. Why, then, does he seek the president’s testimony? It can only be to provide it to Congress for impeachment proceedings.
The House can impeach the president for refusing to testify, but Mueller violates separation of powers if he uses the weaponry of a grand jury subpoena to gather evidence for another government branch.
If Mueller does not have evidence of collusion after 22 months, he should not be fired. He should call it quits.