These stories make me laugh.
Fist, I find it funny that they think that they are SO important that any demon actually CARES if they become a witness or not... There are 7 billion other people on the planet, and only a few million demons. Get over yourself...
Second, I find it funny that they always think that they are SO much smarter than the demons... What demon would be dumb enough to do that? Practically DRIVE you to the religion by attempting to scare you. Yeah, real smart.
Third, why would God allow a demon to get anywhere near you? Like to hold you to the floor for hours? The very fact that a demon did that proves that Yahweh couldn't care less. So apparently the demons are more interested in them becoming a witness than God is...
To answer your question. I don't believe in demons. But if I did, I'd be their ally.