Q57. "I am in the middle of a raging argument about amendments. Can a ratified amendment ever be 'unconstitutional'?"
A. No, with one current exception. By its very definition, an amendment is a part of the Constitution, and hence cannot be not constitutional.
The exceptions are listed in Article 5; nothing baring the slave trade prior to 1808, and no change that denies a state its equal suffrage in the Senate, without its consent (so that a group of states cannot gang up on one state).