How exactly does this new mechanic work? Sounds like the custom power used to model Aristedes' magic.

1. I agree with everything you have to say up until your last point. I don't think that you they have to give the whole truth. They are under a stronger compulsion than usual to answer the question and will therefore probably offer up more information but they can still exploit the way that the question is worded. Remember isn't not that the Sidhe don't recognize the spirit of the question and choose to adhere to the letter, its that they only understand the letter of the question. Also, I think you need some fraction of the creature's name in order to do this.

2. That's right, but that in their nature thing is really stressed. Fetches might go around killing people willy nilly without repercussion but I'd if Lea just iced someone flat out, without provocation, she would probably take on a the price of a weregild because of it.

3. Well the hardest is part is we don't know the duties of each of these stations. It's specifically stated Maeve has duties which she needs to attend to, one that are crucial for Mab to be able to fulfill her's. Largely I think its more of a matter of power, the Knight can choose to disobey any order. In Summer Knight, Maeve has to put in a great deal of effort to place a geas on Lloyd Slate. So in a battle of wills, you have a chance to win against the Lady of your Court, but if you are talking about a Queen or a Mother you have little to no chance. That is why Harry is so flippant with Mab, he knows if she wanted to she could make him do something he didn't want.

4. Kind of answered that in the last response. I would personally discuss it with the player, it would be a compel mechanically speaking, but the difference in the consequences of resisting Maeve and Mab are exponentially different. That could lead to an interesting campaign though. Generally, even though it may be in the nature of a powerful NPC to extort a character in this manner, its best not to let that bleed over into mechanical side of things. Fate and DFRPG are more about collaborative story telling than that.

5. Yeah this is a really hazy line. This kind of choice should be made with the player and GM but the mechanics of the game make it so it is possible for it to happen accidentally. If a RCI pushes their hunger stress too far and have a huge check to make without that aid of fate points you can: let them make declarations to help, allow other players to maneuver/spend FP on their behalf or give them fate points in exchange for auto compels down the line. Mechanically this is what happens when Susan almost turns, it is Harry's player who stops her from turning. So its certainly possible to prevent this kind of thing from happening. I think it would be a really cool thing for a party actually for them to come together after an intense fight to bring one of their fellow combatants back from the brink of losing themselves.

Eh, I think -4 is way too expensive.

Maybe its too expensive for a different way to model things but for what I wrote I thought -4 was a fair pricing, maybe -3.

I've got a PC with Intellectus in Central Park over at RagnarokNYC and we handled it as a -1 Supernatural Sense power and use it to make declarations w/ Lore....

Perfectly valid way to do things, I just wanted a better way to represent the broad range of applications intelluctus can in a more mechanical way. Harry uses the power numerous times in the books to provide a critical advantage in the story. So much so, that in my opinion, it warrants more breadth than just Declarations.

Considering Demesne is a -1 power and lets you do everything you could do with Intellectus plus more, -1 is the most I'd pay for it. Unless your use of Intellectus comes up in every scene, it's definitely not worth -4. Maybe -2 as you have it written....but I prefer just setting it as a Supernatural Sense and calling it a day. Why make things more complicated than they need to be?

That's a good point, but Demesne is only applicable to one specific part of the Nevernever. Unless your adventure never leaves that one place than you aren't going to get too much utility out of the power either. But conceptually the power is very similar to Intelluctus and only costs -1.

I guess it really comes down to the utility of the power as you mention. There is a lot rolled up into what I have written, but if it is barely used than it might not be worth the refresh. It was more that I thought the ability had a broad range of applications that could be better modeled than with just Lore declarations.

So is it over complicating things? Maybe, but I've never run a game with intelluctus or tested this power. But thank you very much for you input 

I agree with pretty much with what has been said so far, expect for the take on his magic. It's made pretty clear that John knows a certain degree of magic. Though magic in the show is shown very differently than in Dresden so I won't say he qualifies for full thaumaturgy either. It seems like he knows a mix of occult lore and tricks than really significant personal power. I'd give him Ritual: Crafting and maybe on Refinement and just have them all as potion slots to simulate his maigcal abilities.

Just purchase Breath Weapon and then Holy Touch and then your attacks satisfy the Holy catch along with dealing an extra point of Weapons damage. It's also fits the flavour for an angelic scion to be classified as holy.