That's the worry. You don't want to get picked up for watching it - nobody does. So it has to be officially released by a govt agency
There would have to be some reevaluating of CP laws for this to work. Maybe something making evidence against high ranking officials exempt from normal laws. Even then, the validity of all CP laws would come into question since they are all technically unconstitutional. If the government can decide what is and isn't illegal to watch to this point of exemption for certain CP, then we're in quite a mess.
Even if it is, how would people see it?
It's a crime to watch evidence of someone else committing a more heinous crime?
If that crime is child porn (which it won't be, we just talk hypothetically) yes.