Ex post facto laws are prohibited by the Constitution so really not sure what the objection to this amendment is.
This is not a law, it's a constitutional amendment, it doesn't need to meet the conditions of what a "law" can or cannot be.
So just to be clear, even though the Constitution states that Congress shall make no law respecting the establishment of religion, it would be perfectly fine for Congress to add a separate amendment creating an official state religion without getting rid of the contradictory bit in Article II, section 3?