Why Mueller should subpoena Donald Trump before the Grand Jury?
Because he doesn't believe a "sitting President" can be indicted. There is no precedent for him to follow. It would be a totally new trail.
But, maybe that is why he should subpoena Donald Trump before the jury? After all, no man is above the law. Mueller would have to possess some powerful evidence to challenge the President of the United States in a Court of Law? Does he have that?
The Supreme Court should be involved in this case. They are the Judiciary. They are the Third Branch of our Government. They should be required to rule on it.
But the only way that would happen is if Donald Trump is subpoenaed and if he challenged the subpoena in the Courts? What other route do we see? The Court should decide.
The problem exists, at the moment, because Donald Trump does not want to be interviewed by the Special Counsel. The Special Counsel is about to run out of patience, in my opinion.