1. I think that when you are dealing with something that is already pretty fantastical, you can either say "all bets are off, anything goes", or try to create a believable scenario around the fantastical elements. I prefer the latter - fantastical elements simply don't relieve you of the obligations of good storytelling. And here, the switcheroo doesn't make sense - it's a lazy gimmick used to deliver one final scare.
2. That, on the other hand, is a good point. However, it would mean that Cochran's ritualistic sacrifice would be more or less a one-shot thing - from him at least.