Unfortunately, it will be a trivial matter for Mueller to obtain a subpoena for Trump to testify, from Mueller's DC-based Grand Jury who have been described as looking like regular participants in Black Lives Matter rallies. And the President will have to respond to that, or plead the 5th, which infers guilt. That's why there are already reports out that Trump's team is now negotiating the terms, which is exactly what Trump is doing with these statements to the press.
So even the President can't run out the clock on a special prosecutor assigned to investigating the President. If Slick Willie had to testify to Starr, then Trump will likely have to testify to Mueller, if Mueller presses it.
Trump should copy the Hillary defense: "I don't remember. I don't recall. I'm not sure . . . . "
“Unfortunately, it will be a trivial matter for Mueller to obtain a subpoena for Trump to testify,”
Tell us more about this ability to use a subpoena to compel a target of an investigation to offer testimony. I hope you at least enjoyed the Cracker Jacks there in the box with your law degree.