You're in good company. But what on gods earth is Copromancy?

Heh. Look up coprophilia, and shudder in disgust. 

Thanks for all the input, folks! Keep the debate going, I like seeing the range of ideologies going on here! 

I was going to go for "you have to gaze through the crotch of it to see what would normally be viewable from his pants"

Then if they don't specify what direction they're looking through the pants have them look the "wrong" way.

And if they look the "Wrong" way, they see what would normally be seen if the bad guy were wearing the pants and unzipped his fly?

Slinging a spell is such a 'fast' thing, you would really need to ready yourself for it to even have a chance of countering it, then you'd need to guess the power that went into it. Everything Harry throws a "Fuego!" it isn't the same, sometimes it's more powerful, sometimes less (not talking about a rote spell, of course), so how would you sense the amount of energy going into it? Not mechanics-wise, but narrative-wise?

With magic mostly occurring at the speed of thought, I would think sensing an incoming spell would be as fast as being able to fling your relatively slow body out of the way of one. As for being ready for it, having held your action may provide the readiness required. I believe the rules even state that the assessment action for determining the power of a spell to be countered is a free action, which suggests that counterspelling could be done in combat instantly.

As for narrative-wise, you should be able to sense the amount of energy going into a spell the same way you would be able to sense the amount of power in a (perhaps thaumaturgy)spell that was cast long before and still is active. I mean, it's not like you can see a spell, in either case.

As far as "instantaneous" spells go, I agree - difficult if not impossible to counter.  Though I think timing is the real difficulty (not certain there's even a way to interrupt an action mechanically). 

Leaving out (most) Attack spells, Blocks and Maneuvers last one or more exchanges and should be viable candidates for countering.  Blocks in particular.  (Had my "angry ghost" locked down by a block last session.    )  I see some advantages to including a sorcerer capable of counterspelling in a larger combat...

I think Mechanically you can only interrupt an action by having a higher initiative and having held an action from earlier in that specific exchange.

Another question: The counterspell rules mention redirecting spells. Even if instant(attack) spells are off the table, Is it possible to co-opt a spell and change its target? It would be nice to take the shield spell off of an enemy and use it myself, or move it to protect an ally who has no magical defenses.

What if the current location is a more or less empty hallway, where hiding stuff is nearly impossible? In this case it ought not to be a question of whether the bucket can be seen or not, but rather whether there is a bucket or not (and using Alertness for that just sounds wrong).

I think in that case, you would have already described the scene as an empty hallway, and you would be able to tell the player that the room has a "Totally Empty" aspect, and hand him/her a fate point. Now, if he were to look in rooms off of the hallway, he might be able to make an Investigations Declaration at that point.

Perhaps an intermediate rule would be better, where a character cannot be taken out physically by the character inflicting Physical damage if most of the victim’s consequences are Social ones.
An example: Player A (Punchy McStabsalot) and Player B (Prettyboy Floyd) are fighting Enemy C (Phineas Mook).
First round, Prettyboy delivers a deep insult by walking over to Mook and spitting in his face, dealing 2 social stress. Punchy whips out his 2-shot Beretta and puts a round in Mook’s foot, dealing him 4 physical stress.
Next round, Prettyboy Floyd hides behind a table, starts in with a tirade of insults the likes of which are never seen outside of a Friars Club Roast, and Mook needs to take a Severe (shamed beyond belief) and Mild (Distracted) Consequence. Punchy goes next, shooting Mook in the shoulder, and Mook takes a Moderate (Ow, a Bullet Hole!) Consequence.
The final Round Prettyboy holds back mid-insult-rant and holds his action, while Punchy tags the Distracted aspect and throws his gun at Mook, Nailing him in the face for too much stress for him to handle.
The GM declares that Prettyboy can detail the Coup de Grace, which HAS to be related to the Social damage, not the Physical. So instead of dying on the spot, Phineas Mook drops his weapon and weeps like a baby, apologizing profusely.

That makes more sense to the narrative than Punchy getting to kill Mook when most of his injuries are social in nature.

Of course, this may be overly complicated...

Alternate usage:
What if you wanted to perform a spell that removes the fire from all of the burning minions (and maybe the scene, as well) and turns it all into a huge fireball to hit the big bad? It could be argued that because the spell is meant to make magical "contact" with all the burning minions, that all of those minions' "On Fire" aspects can be tagged. Would this be something to allow?

"This power is not necessarily absolute; plot-device-level threats still have the potential to harm the creature even if the catch is not satisfied (e.g. a nuclear bomb, the combined magics of the entire senior council, etc.)"

That's pretty much the same type of thing I was about to bring up: Being the GM, you have to understand that all powers as written in the book have at least a little bit of "wiggle room", where the Story's needs are concerned. It's probably a good idea to imagine that every rule has a "PS: this is subject to the needs of the GM and the Story; Don't be ridiculous with it," invisibly written after it.

As for Physical Immunity, I think it can be fair, but only if the GM is mature enough not to abuse such a power. The Original Poster mentioned the possibility of a nuke, or the entirety of the White Council aiming a ritual at a character. I wonder, though, if they intend for this to happen in the course of their game? If not, then the question is moot. If you can only come up with improbable, ridiculously powerful things as examples, you're stumbling into the land of hyperbole.

As several posters have mentioned, the reason to choose Physical Immunity rather than Mythic Toughness and Recovery is because it fits the character you're making, the experience you want your players to have, and the story you're trying to tell. If you want a baddy who takes a licking, and keeps on ticking, but gets visibly worn down as the fight progresses, Take the Mythic coupling. If you want a villain that seems to be unstoppable, until you figure out his Achilles' Heel (Perhaps literally, if you have the players come up against Achilles), then he goes down quickly when hit with "the right stuff", then choose Physical Immunity. It's the story that determines whether the power is appropriate or not, not just the Letter of the Law.

And if you are allowing Physical Immunity for a Player character, I suggest you have a sit-down with them regarding how you view the power, and the restrictions you will be putting on it, if any. Perhaps requiring an extension of the Catch, so that the character can be hurt by, for example, "Left-handed Sword attacks and single attacks of more than 12 shifts of damage," would make it more palatable to both parties.