This was not a Bill of Attainder or a law, ex post facto or otherwise. This was an amendment to the Constitution itself.
But even then Amendment XIV states, 'But Congress may by a vote of two-thirds of each House, remove such disability.' The 14th was not a hindrance to his trial.
It most certainly was. The disability was in place and Davis was forbidden, because of his leadership in the rebellion, from holding office. A trial and punishment would have been unconstitutional, thanks for the 14th Amendment.
BS. The Constitution is the supreme LAW of the land. And a law existed that prohibted ex post facto laws. Until repealed, it is the law.
The section in question was ex post facto - it criminalizes an act committed before the passage of the 14th - which was not illegal at the time - and punishes such action by loss of civil rights. Additionally, it ludicrously overlooked Amendment VI which required that the accused have 'the right to a speedy and public trial, by an impartial jury of the State and district wherein the crime shall have been committed, which district shall have been previously ascertained by law, and to be informed of the nature and cause of the accusation; to be confronted with the witnesses against him; to have compulsory process for obtaining witnesses in his favor, and to have the Assistance of Counsel for his defence.'
A trial and punishment would have been unconstitutional, thanks for the 14th Amendment.
'3. No person shall be a Senator or Representative in Congress, or elector of President and Vice-President, or hold any office, civil or military, under the United States, or under any State, who, having previously taken an oath, as a member of Congress, or as an officer of the United States, or as a member of any State legislature, or as an executive or judicial officer of any State, to support the Constitution of the United States, shall have engaged in insurrection or rebellion against the same, or given aid or comfort to the enemies thereof. But Congress may by a vote of two-thirds of each House, remove such disability.'
All congress had to do was vote to REMOVE the 'disabilty' with regard to President Davis. Then the trial could commence. There was no legal hindrance to the trial of Davis.