There's no way some escaping (or otherwise too much tortured) witches wouldn't press the third button just for the heck of it when they are running out of drugs or just out of their endurance and wits. It's a stupid idea, no matter how you look at it. In the first place, the organization has no use whatsoever for such functionality. They dispose of witches that turn out too weak, fail a mission, or for any arbitrary reason. They ever remove all evidence the witches exist by altering memories or killing witnesses. So, why would they have a button to free witches? It would be a huge liability with no benefits.

The already dead theory has many merits, though. But then again, we have already seen they are already dead as humans in any case. Humans don't turn into goo in seconds.