We do not know much about the actual mechanics about tempering, so there really isn't enough information available to answer your question - unless there is something in the Lorebook.
We do not even know for sure that tempering actually is irreversible, only that people in the game believe it is.

What we do know is that nobody in-game knows how to cure it, and we can safely assume they have spent a lot of time studying this.


Thancred is not nearly as casual about the executions as he may appear. He knows full well it is a terrible thing to do, which is why he prefers we hear it from him rather than from some random person.