So is he after revenge, or protecting mankind?  I ask this because striking bargains with any "dark forces" (Black Court being a great example) seems a lot like aiding and abetting the folk who mean humanity harm, which is kind of the opposite of protecting mankind.  If he wanted to avenge the deaths of his family, and was "flexible" about the means used to reach that end, then it would make more sense to go that route.

You cannot price a Power based on the assumption that it does not do what it says it does.

And yet you seem unconcerned that with the inclusion of TINS, ACAEBG no longer does what it says it does?

So what happens when somebody builds a new custom power, maybe "All Creatures Are Equal Before God's Might", which has exactly the same description ACAEBG does.  It would work against TINS because TINS doesn't grant immunity to ACAEBGM.

Meh, the whole discussion is silly; go ahead and design your power as you see fit, I'll step out.

Two, really, since you can't spend your last refresh.

Have you considered a Sponsored Magic?  For example, you could replace Domination with Kemmlerian Necromancy (no net refresh cost) and be able to use Psychomancy to do Domination-like stuff via Evocation.  (And then some.)

Also, you probably need a Lawbreaker or two, if you've been naughty.

Don't know if this really is what you're looking for, but I'll throw it out anyway.

First we start with the sponsor debt mechanics (ie, what admiralducksauce suggested).  This gives you a solid core for asking for little bits of help -- for a price.

But take it a step further.  Allow the character to make extensive use of the "Temporary Powers" sidebar on YS92, but using debt rather than Fate points.  Work with the player to make a list of powers that are appropriate to borrow from a demon.  This should certainly include the physical powers (Strength/Toughness/etc), and might well include a number of others (Cloak of Darkness, Hellfire, and more).  Basically, for a future favor (or favors), the character can access -- for a scene -- as many of these powers as he's willing to pay the price for.

And I'd consider adding just one more small caveat, if you think the player would be cool with it.  Pick a reasonable number in advance (perhaps trouble comes in threes?).  Once the character has "borrowed" a particular power that many times, it sticks.  He adds it to his character sheet permanently, reducing refresh and losing Fate points as though he'd just bought the power.  He's just lost a piece of his soul, and become a little more like the demon he's dealt with.

In effect, the result would be a somewhat Changeling-esque character, who struggles between the power on the offer and the desire to retain his soul.  Of course, if his refresh reaches zero, the Choice is made.

Someone with any form of spellcasting that included focus slots can create potions, and those potions can do [mumble, mumble-mumble].  Er, the book isn't clear as to what limitations are imposed on people who have less than full wizard abilities (Evocation and Thaumaturgy) but who have slots.  Some people think focus item slots are focus item slots are focus item slots.  Others think that focus item slots can only be filled with items that reflect the caster's powers.  So can a character with only Channeling (Fire) -- which includes 2 focus item slots -- use those slots to own a Dresdenesque protective duster?  The RAW doesn't say no, which is typically interpreted as a yes.

But back to your question, which I think was "can a potion slot be used for an 'offensive' spell?"  If you look at the sample "potion" list on YS303+, you'll see several examples of just that -- "potions" that generate attacks -- and aren't necessarily actual potions at all.  For example, Ghost Dust and Sunburst in a Hankerchief.

What would happen to the Autumn Knight if the King dies? Obviously, the Prince would be promoted, but I think that Lily being promoted to Summer Lady was kind of a special case. Would someone else take the Autumn Prince role? Or could it throw things even further out of balance by leaving that slot open?

Just as an FYI, WoJ notes that there has been one replacement of Mother Summer during recorded history (Mother Winter has never been replaced).  I thought I remembered reading that there had been several (three?) Summer Queens, as well, but I can't locate the reference.  So rare, yes, but not unheard of.

Also note the following WoJ:

If the faerie queen dies, do you get out of the contract?
You just have a different faerie queen show up to… the faerie queen is an office, it’s not a person, Lily found that out the hard way, you know, so…

I think that that would answer your question about the Autumn Knight -- his leash would pass to the new King.  Unless the new King didn't like him, of course, in which case he could be replaced as desired.
Essentially abdication.  The previous mother wearies of her duty and moves along.  There's been one new Mother Summer during recorded human history.  Mother Winter has never retired.

a jeweler's magnifying eyepiece (don't know the technical name)

Jeweler's Loupe is the term you are looking for, and I kind of like that idea...

As long as the cops are pure mook-grade NPCs and they concentrate themselves into a fairly small number of zones, then even a fairly modest evocation is enough to take them down.  For example, if you define "the front yard" as a zone, and "the back yard" as a second zone, and stipulated that the 200 cops were packing themselves into those two zones to get into the house, then all you would need is maybe a 5 shift evocation (assumptions: weapon:1, 2 zone spell with control of 7 or so vs cops with armor:2, defense or perhaps 2, and no consequences).

However, I think that this is poor application of zone rules, and downright silly tactics on the part of the cops.  My thinking is that every half-dozen targets, tops, should constitute a "zone" (redefining the zones for tightly packed masses as needed -- ), and that in any case the cops would be fairly spread out even without this sort of adaptive zoning, enough that there would probably be fairly few zones with more than a handful of cops packed into them.  So the 200 cops might require a 30-40 zone effect, which is ... non-trivial.

Sometimes you need numbers.

Agreed.  But I think that Richard is trying to say is that sometimes it's best not to have numbers -- or that sometimes the numbers aren't relevant.  And I agree with that.

Say, for example, a group of PCs learns that there is a nuclear warhead headed at them.  It is valid for the GM to dictate that the only way to escape the blast is to not be in the fireball when it goes off (run like mad, escape into the Nevernever, or whatever) -- and that a magical shield of any strength that can be produced in the time available just won't cut it.  In such a case, putting numbers to it is just throwing down the gauntlet for the rules lawyer in the party to prove you wrong.