They would not turn the worst of these terrorists loose....you are missing the point. They are simply talking about turning Gitmo loose as a decision completely separate from the decisions of what will be done to those terrorists there.
Some will be tried and executed (military courts) some will simply be moved to other prisons (Stan, Qatar, Iraq and the like).
Your "scenarios" are full of too much Fox News-type "Infotainment" items and ridiculous assumptions about what our administration can and won't do. Despite your use of acceptable buzzwords, you are off the mark on so many things it's hard to sort out and know where to start.
So I won't.
Inspite of all your talk, you are missing "key facts". ..the ones that provide the "glue" that holds the world picture together. You have the elements on your table. You just have'nt connected them.
Think.
Starters: The Dems are dead in the water.
As a lame duck President, whose party just lost Congress, what would he have to lose politically? It doesn't take an act of Congress, although we have one dated September of 2001 authorizing "all necessary and appropriate force against those nations, organizations, or persons he determines planned, authorized, committed, or aided the terrorist attacks that occurred on September 11, 2001, or harbored such organizations or persons, in order to prevent any future acts of international terrorism against the United States by such nations, organizations or persons.", for the President to call over the officer with the nuclear football, and let fly. A single Ohio class submarine's load would be overkill.
That same resolution also declares that "the President has authority under the Constitution to take action to deter and prevent acts of international terrorism against the United States", so even if you argue that Iran was not involved in 9-11, nor aided, nor harbored (they did lots of harboring) those who were, the President has the inherent authority as commander in chief to do whatever it takes to protect the US against further attacks.