1. From everything I've read, full-fat dairy would be better. I'm annoyed with that, because I LOVE Fage Greek yogurt, but no one anywhere around me carries anything but the 0% either. I did find full-fat Greek Gods yogurt at one grocery store, which isn't quite as good but close enough. I wonder if your grocery store would carry it if you asked them to.
2. If I remember correctly, stevia doesn't spike your insulin like other sugars, but I think there is somewhat of an insulin response to anything that tastes sweet. I'm not sure about that though. For me personally, I wouldn't consume anything that tastes sweet during a fast, but you might have different results. Try it. If you crash, then you know the answer.