Police tactics do not apply to civilians. The reason Police don't shoot IS because they control the situation. They secure the perimeter, which keeps the hostage taker from going anywhere, then they wear him down psychologically to get him to surrender. If someone is in your house with a family member hostage trying to leave, that is a totally different situation. I'd like to see evidence that supports the idea that letting a BG leave your house with a family member as hostage is wise.
You postulated 50% success rate from what I wrote. I postulated 100% when I wrote it. My point is that no one can tell you what will happen when you shoot someone anywhere, not just in the head. But, I would be willing to wager that fatal or not, the hostage taker will not be even thinking about shooting the hostage if you shot him in the head. His first thought will be the realization that you just shot him, at which point in time the second round would be hitting him.
No one can guarantee that the BG will kill the hostage or not if he leaves either. But you can guarantee that the hostage will now be left to his or her own devices and at the mercy of the BG's whim. For me, that is not acceptable. Having said that, I need to reiterate that if you don't know that you have the ability to make the shot, then don't.