Well, rest assured, that's already the case in RAW, there's a whole text about it in RoP: tD: Holy men can be tempted, even lose their powers temporarily, even lose all protection from demons, all as a test of their faith. I can very well see the God putting a character's faith to test by putting him before tough choices. And devils might put him before (different) tough choices, too, like destroying a mine that make town dwellers greedy, or condemning a whole town to poverty (Unless I'm wrong, the correct choice theologically speaking would be #1, but I'm not even sure)
Maybe I don't understand what you mean, but I doubt very much that gaining the free power to destroy angels would change anything to that, any more than, in RAW, having or not DEO changes anyhting to these.
That's an interesting point, since I agree that DEO can, in fact, be an effective "Demon Detection Spell".
The reverse, however, is that, if our pious magus has to be careful when casting DEO for fear of destroying an angel, it'll also stop him right in his tracks on the Pride path (Substituting himself to god and his angels, feeling almighty, you know the deal).
Both points are (IMO) rather moot, though, since the cases where you cast DEO are already relatively few and far between: You won't cast it on every mundane you encounter (unless you want to alarm them), and I don't think there'll be much cases of demons masquerading as angels.
I'm thinking: Maybe you want to slip us a demon and fear he might be detected too easily? Remember then: There's always possession 
And human (or hermetic) agents^^
You can also have faeries posing as angels or demons (And evil faeries, too), and I can perfectly see a magical/faerie creature having being compelled or having entered a bargain with a demon and forced to obey him 
But to tell you the truth, more than anything, this makes me uncomfortable because of the "free lunch" for the common PC magus (the one that would have no qualms into basting an angel that opposed him) and, more importantly, as something too modern in thinking that destroys my sense of "medieval world". It'd feel like hanging with the top 5% of the black metal crowd (the ones that reject any supernatural dominion over man), and I'm already there 8)
I'll have to check the spells, if I think about it.
As for the Impassive face, well, that was Callen's idea, and I think it's a good one: Going with flavorful items make things easier than the RAW option 2, but not totally, since you add free requisites as needed by the effect.