“If the Supremes voted against me, I’d plead the fifth and make the courts fight it out.”
Even if the Supreme Court says they have the right to subpoena you, there is still no enforcement mechanism to make the President testify. You want to charge him with contempt? Who is going to do that? Oh yeah, the DOJ, who works for the President. You want to haul him to jail? Who is going to do that? The FBI or US Marshals, who also work for the President.
Just do like Obama did when Congress tried to make Holder talk. Ignore them, because there are no means (short of impeachment) to enforce anything against the Executive Branch unless the Executive chooses to cooperate.
I am not a big fan of your strategy because even Executive branch law enforcement agencies are going to follow court orders. But what people here are forgetting is that Trump is Mueller’s ultimate boss and can fire him any time. The old Independent Counsel law was something else; Mueller is just acting with the power of a US Attorney per DOJ regs. So DJT can play this out and see what develops.