And if those provisions were ever to find themselves in conflict with Article VI, they would be invalid.
More neo-confederate mythology.
Here's a link to all of the Texas Annexation Documents Why don't you show us the clause that gives Texas the right to secede. The only right Texas had in annexation that other states did not was the right to break itself into as many as five states if it chose to and Congress approved. Other than that, it was no different than any other state.
As for the oft-mentioned Virginia 'right to secede' if you study the ratification document, you will find that it, like New York's, is a conditional ratification of the Constitution based on the future inclusion of a Bill of Rights. That Bill of Rights was inserted making that clause moot.