It might actually be a trivial task to withdraw the said ETC if The DAO THIEF doesn't feel like creating further problems, but it's not guaranteed that he/she won't.
That's why I wanted to know if the "white hats" have expressed any opinion on this. My understanding is that even though the coins are in a child DAO controlled by the "white hats" it would still require quite a bit of effort to fully secure those coins and success is not guaranteed. Probably not quite what the OP is claiming:
No, the solution is very simple.