The official position of the Union is that they do not get to decide. The Union decided that they remained citizens of the United states, and so therefore the Union should have been forced to treat them like citizens of the United States.
If the Union acknowledged they were in fact citizens of another Nation, then what they did to them can be excused, because Constitutional law does not apply to the citizens of other nations on foreign soil.
But that is not the claim the Union made.
Your batting 0 tonight. The official position of the United States was that they were insurrections/rebels, insurrections/rebels have little rights and can be hung.
Get back to us when you have made a serious study of the law of rebellion and insurrection.
What court decisions support your proposal?