We actually don't know that Harry required the permission of Mickey's wife. We know that he *felt* that he needed that permission. And while I normally wouldn't be that pedantic, I think it's important to this topic...
Magic comes from within and is a reflection of who your character is at a fundamental level -- thus the "Lawbreaker" reflecting that your character, deep down, believes magical murder (for example) to be justified in at least some circumstances.
HARRY believes that he needs Mickey's wife's permission to enter the house with the full use of his powers. Therefore, for Harry her permission is required.
If the Scion Lawman mentioned above truly and fundamentally believes in the law and that it gives him permission to persecute offenders wherever they go... Belief and free are such an intrinsic part of this system that I would allow the argument that a legal and court-ordered warrant could bypass a threshold for that character -- *IF* the Lawman truly believes that it is a legal warrant. I would also heavily restrict him if the warrant was obtained through shady means, on shaky evidence, or if he knew that the warrant wouldn't hold up on review.
I kind of think this is akin to breaking an oath sworn by your power -- the lawman's faith in the law is giving him protection from hostile thresholds... but only when the law is really giving him the right to enter that home.
Obviously, your mileage may vary, so I think this is a DM issue.
No. Harry needs permission because the whole point of him coming over is to use magic, and if he just walks through the threshold, he leaves his magic outside. His argument for an invitation is because he can't just say, "Invite me in so I can do magic on your devote Christian husband."
You are wrong because if you are right, then all monsters have to do to ignore thresholds is convince themselves (1) they don't need invitations and/or (2) thresholds don't exist. And since they can't do either, thresholds matter.
Facts of Thresholds (not inclusive):
0.
Without an invite the following facts are true. An invitation by a person living behind the threshold grants exception to everything that follows.
1.
Thresholds never hedge out mortals or purely mortal things. A non-mortal can throw a firebomb into a home just fine. A non-mortal can try to violate a threshold but then they have to contend with whatever your GM feels is a fair wet blanket.
2.
Thresholds do hedge out mortal magic. A mortal practitioner can walk through easily, but if they want to do their shtick (magic), they need to try really hard or get the invite first.
We haven't seen how thresholds effect mortals with special powers (Kincaid has never been scene violating a threshold). The closest was when Susan stepped over Dresden's, but she didn't do anything then. When she tried to vamp out it was later, after Dresden invited her in (I think it was more of a "Hurry up and get inside!").
But since vamping out would lead to bad things for Susan, it makes me think that her powers being dampened would be beneficial, so I would say maybe use the wet blanket approach (instead of the outright "no supernatural powers").
If you are behind a Fair threshold, increase all difficulties by 2 (defenders get a free +2 on their rolls as well). While the initiative bonus of Inhuman Speed might still be advantageous, taking an effective -2 (or more) on all actions can suck (-2 on Might when trying to pin and apprehend a suspect?).
So a Lawman with a bit of the Nevernever in his veins is better off not using their mojo when operating behind a threshold. Or get good at Deceit so you can get an invite before you go "Freeze! U.S. Marshals!"