Just recently got into this sort of conversation. Temporary vs permanent is definitely an important distinction. But for the most cases, it's going to be permanent.
"Your Story" mentions that a particularly strong willed person may be able to keep their mind intact for a while after transformation. So (s)he may be able to communicate in very rudementary ways. But it does destroy their mind sooner or later, and usually sooner.
Maybe your PCs run into the victim while he's transforming. A smart/skilled PC could quickly place a ward around his mind to prevent him from losing it. Or maybe the ward just slows the process giving the PCs hours?, days?, weeks? to find a way to remove the curse or track down the warlock and kill him to break it. (26shifts min to permanently transform somebody, vs maybe 10-15 shifts impromptu). Or maybe its just temp because of the ward.
BUt generally speaking, permanent is mind destroying. And as for changing back, technically, no it's not a violation. The general theme/feeling/"harry's technicalities" (because he uses it often in the books) is that you are not transforming a human.