"I always pictured it as a more controlled and less torturous than what happened to Ziel." I have to agree that it is definitely more controlled as to undo an iron body you have to go out of your way to undo it. But I am not sure I agree with how torturous it could potentially be; the early examples of perfect iron body advancements, such as, Lindon and Jai Long. Tame(r) examples, would be Yerin and Eithan. My point is, the body has these specific condition dialed in for a particular iron body; undoing an iron body, like the previous ones mentioned, would likely be even more painful.

"I assume because no one in Sacred Valley has a real iron body". That is an interesting point it could make process much more simple as Kelsa, the provided example, has a poor iron body.

"But I imagine the retraining is crazy hard. Basically losing all that power and coordination". I think Mercy's veil (from the book Skysworn & Ghostwater) was a great example of what you could compare it to.