regarding demonic encounter, i would think about what kind of reaction you'd want from the entity. the act of observation almost always changes the thing you're observing: unless you're invisible? that seems like it would work, although i've never been face to face with a demon. but if you're invisible on all planes it should ignore you so you can observe it in its natural habitat. i think you'd have to be careful to stay neutral enough to preserve the construct so the entity doesn't figure it out.
if you want to want to exorcise or even heal the demon, start radiating pure love. at the very worst it will be afraid of you. i think that's what evil really is, total fear for one's self. very self-conscious creatures, demons.