“...SCOTUS is a co-equal branch of government and is not obligated to recognize or submit to a congressional subpoena.”
Is not the Executive a co-equal branch? Is the President obligated to appear before a grand jury?
“Is the President obligated to appear before a grand jury?”
In my opinion no, but politically it might be viewed poorly if he didn’t.
I would assert Executive privilege. Mueller works for Trump and could be fired by Trump. Congress could then impeach Trump.
I guess everyone is carrying an ugly stick; it’s whether or not they have the nuts to use it.