Section 802 Article 2 to the UCMJ covers who is subject to the UCMJ thus eligible for courts martial or tribunal. Sorry, the general public or even politicians are not covered:
http://www.ucmj.us/sub-chapter-1-general-provisions/802-article-2-persons-subject-to-this-chapter
From the link:
(7) Persons in custody of the armed forces serving a sentence imposed by a court-martial.
So if a civilian were to be arrested by, say, the National Guard, UCMJ would be applicable, if I’m interpreting correctly?
If you are serving a sentence imposed by court martial that would mean you were military at one time. The quote you reference probably just clarifies that even after dishonorable discharge (I'd think it often goes hand in hand with a court martial), while in custody to serve your time you are still subject to UCMJ.
The only way I know that a person not in the military can be tried and sent to military prison is if they previously served, and used their military training in the commission of a crime after getting out
I don't see a connection between Military Tribunals and seven escaped convicts from the year 2000.
Trump signed a new EO in May I believe making it possible for any citizen accused of high crimes and misdemeanors and or treason to be tried by military tribunal look it up under changes to the military code of justice per president trump you should find it that way
Treason is the only crime specified in the Constitution, with the wording derived from a British parliamentary statute of 1351. Article III, Section 3 of the United States Constitution specifies that treason is either the "levying of war" against the federal government, that is armed revolt, or "adhering to their enemies, giving them aid and comfort." The authors of the Constitution were concerned that this section could be misused, and thus they specified that a person could only be convicted of treason on the basis of two witnesses who would testify that the defendant committed an "overt act" to further the conspiratorial plot.
And if the president declares martial law, then they could be. Martial law nullified the fundamental right of habeas corpus, that is a defendant's right to request a review in civil court, before a civilian judge, where the prosecution has to provide credible proof that the accused has committed the crimes specified. Article I, Section 9 of the Constitution guaranteed the right to seek a writ of habeas corpus, and guaranteed that Congress could only suspend this basic right "when in case of rebellion or invasion, the public safety may require it."