See, I don't think we can really answer the question "does a mortal using a sponsor's magic to break the Laws assign the Lawbreaker power to that mortal?" because we don't know exactly what Lawbreaker represents, and how it comes into being. 

If it is purely a stain on one's soul, 100% internal to the person using magic, then any mortal who breaks a Law purely with sponsored magic should get Lawbreaker.  The reason I don't think this is the case is precisely because subduing someone with magic and then shooting them in the head is not a gray area at all, the Wardens do it regularly. You've still used magic to kill, it just wasn't the actual instrument you used to deliver the coup de grace.

If Harry is right, and magic really is the essential force of life and happiness and puppies, then breaking the Laws is perverting the nature of magic, and Lawbreaker is the backlash of that act.  It's an outside force damaging the soul of the user.  In this case I do not think that a mortal using purely sponsored magic would become a Lawbreaker, because the power in question is not the power of life, but the power of some other entity, generally one of the Faerie courts (since most other types of sponsored magic require the user to have magical power of their own). 

The gun analogy doesn't apply since it isn't the act of killing that is problematic, but the act of killing with magic (or breaking one of the other Laws).  Unlike the "guns don't kill people" argument in this case the instrument is absolutely the issue.

Yeah, until the NPC buys off his negative, he has no FATE points. If Niccodemus is compel, sure he gets a FATE point. Now he's only 17 in the hole, rather than 18 or whatever.

That really isn't supported in the text.  NPCs with 0 or lower refresh don't start the session with fate points, but the section on NPCs and fate points goes into several examples of such NPCs earning them through play.  If you want to houserule that negative refresh NPCs start with negative fate points they need to buy off before usable ones will accumulate that's fine, but it's still a houserule. 

Good point.  He could always be a Freeholding Lord, or for all we know he might have signed on as head of the Goblin Nation (or something).

The real question is: how would his inclusion help Mab?  Can you think of a way?  Then he's probably in.

The real problem with telepathy is that it violates the laws of magic, so if you use it the wardens will try to kill you.

Not necessarily.  Consentual mind-to-mind communication is perfectly kosher, .  

A couple options for making this sort of psychic playable:
1) It could be possible to pick up a person's surface thoughts without invading their minds, maybe they somehow affect the aether/mana-sphere/whatever in a way the player can read.
2) Include a certain amount of precognition.  Sure it might butt up against the Sixth law, but as long as the PC sticks to the gray area it should be points well spent.
3) Be sure to throw plenty of inhuman foes at the players.  Your psychic can go ahead and mindrape vampires, faeries, and other non-human sentients at will.

Your co-GM needs to learn to say yes to the players more.  So long as no one is being immature and attempting to break the game there's really no reason to turn down a player's concept.  If I was worried about anything when I picked up Dresden it's that the characters would have too much weight, and no one would want to play anything/anyone original (ie. we'd have a Harry clone, a Murphy clone, a Thomas clone, etc.).  You and your co-GM got lucky in having a player who wants to break some new ground.

No, a tag is a one-time only cookie for discovering or creating an aspect, further bonuses or rerolls require a fate point.  On the other hand, there's no reason not to compel the crap out of "blinded" or a similar aspect any time an affected character wants to do practically anything.

Or you could take Giant Meat Paws (or Fist Like A Canned Ham, etc.) as a derivation of the Claws power.  It's damaging natural weapons, they don't have to be sharp.

Yes, but I think that Janus is now thinking about a house rule now.

Personally I'd consider charging ubernegative NPCs extra fate points to buy out of compels. 

One thing I want to make clear, when I talk about an NPC with negative refresh, I mean an NPC that has spent more points on powers and stunts than it can afford with its base refresh.  It isn't so much a balancing mechanism (total points spent on powers/stunts determines actual power) as it is a measure of how much control the NPC has over its nature.

Think of a character who picks up a Denarius while at a +1 total refresh.  At first he resists the Fallen and maintains his positive refresh.  Eventually our tragic anti-hero gives in and buys Hellfire at the angel's urging, bringing him to -1.  Sure, he's an NPC, but he isn't going to go around slaughtering his friends and family for no good reason.  He might even help out his former comrades from time to time.  As time goes buy the Fallen helps him master his magic, and he buys four more points of refinements, for a total of -5.  At this point he's pretty far gone, but still recognizably the same person he was.  Twisted and evil, yes, but still the same person.  Then he goes hog wild and takes all the shapeshifting powers, bringing him to a total of -15.  Ouch.  At that point I'd say the person he was is gone, entirely under the thrall of his Fallen.

Come to think of it, I'd say that the Denarians who keep their human names probably don't have substantially negative refresh totals.