True, with the sole exception for those who, having been under oath to support the Constitution of the United States, gave aid or comfort to the enemies of the United States, or participated in insurrection or rebellion against them.
"In submitting the resolution for the amendment, 28 Senators had been unlawfully excluded from the Senate in violation of Article I §3 and Article V. 42 Representatives were similarly denied seats. 15 of the 37 states rejected the Amendment."
Each house of Congress, as I'm sure you know, is the sole judge of its membership. There is no appeal to that decision, and therefore, no lawful claim can be made otherwise. This confederate mythology about the 14th Amendment has been thoroughly refuted.
"Did the amendment CRIMINALIZE their actions?"
The traitors actions were criminal in their own right. The 14th Amendment provided a disability for their treasonable conduct. The amnesties offered by Presidents Lincoln and Johnson had saved more than a few of them from the hangman's noose.
"It's not hard to comprehend."
What is not hard to understand, except for the exceptionally stubborn or the willfully ignorant, is that NO constitutional amendment is itself, unconstitutional. When proposed and ratified, it works to change the terms of the existing Constitution and Amendments, absolutely. That's why the process is exceptionally cumbersome.
No, they weren't. If they were, where was the trial? Other than the hanging of Wirtz, over Andersonville, you can't produce the name of a single general or field-grade officer of the CSA who was tried for treason. The U. S. Government held Jefferson Davis for years and didn't try him.
Your saying it doesn't make it so. The Confederates changed their allegiance openly and supported their People in what they wanted to do, which was to leave the Union and found a new republic. That isn't treason, and neither is levying war against the United States treason, for anyone who has openly renounced United States citizenship and declared for another sovereign -- or for no sovereign. These people were citizens of another country, and not citizens of the United States. The partisan and clearly expedient declarations and decrees of the United States Government and its courts to the contrary notwithstanding.