Well, when Harry uses it as "mystical rebar" that's sponsored magic.

That's kind of my point. Sponsored Magic as 'boost' is kind of the edge case, but that's ALL Soulfire seems to do (according to the novels). And while the ability may or may not actually be granted by the Almighty (as opposed to having been simply unlocked) it feeds on the 'soul' for power rather than some sponsor with an agenda. That's why I think its a really bad model for it. Anyway, not gonna get in a debate about, just voice my opinion.

When you say Discipline vs Might/Endurance, do you mean that if your Discipline roll beats their Might/Endurance roll they move 7 zones?

All or nothing, yeah. Only other thing I can think of is adding the shifts to your roll and subtracting the resist roll, which seems more likely to be broken.

The Intimidation item spends 2 shifts to become zone-wide, which is weird for a skill replacement item.

Fair point, I guess. Honestly it feels like the sort of thing magic should be able to do but I wanted to avoid an offensive mental evocation. Actually it feels like the sort of thing Intimidation should be able to do... I'll look into that and see if it makes more sense to rewrite it.

The flashing quartz also spends 2 shifts to hit a zone, which when added to its power of 4 leaves one shift left over.

I think I just used the wrong word level. Its basically just a magical flashbang, anyway.

The reflexive defence items, like most reflexive defence items, are automatically a little bit questionable because the rules never specify which defensive items can be activated reflexively.

YS280, "Defensive items (ones that provide armor or a block, for example) often consume a use at the time of defense and don’t require a separate action to activate." It's up to my GM but they are providing a block. Arguably you could set them up as a trigger on a Mediocre Ward or something like that instead.

I've been keeping him under control by throwing lots of mortals into the fray and compelling him to not use weapon 10 nukes for fear of accidently killing someone.  But when we fight "supernatural" creatures, it's kind of a joke, really.  He one-shots creatures fairly regularly.  Except for the few enemies dodgy enough that he can't hit...which is probably the biggest draw-back of a crafter.  It's hard to get your accuracy up there like you can with a straight up wizard.

I see three ways of looking at that. One is your players have basically said that they don't really want the game to be a challenge. Another is that they want to face enemies who are tough enough to take (or avoid) those 10-stress hits without a problem. Or, they're asking for the challenge to come from another direction entirely.
Also, remember zone-wide attacks are NOT selective, so forcing the players into a zone with borders (so they can't move out and act at the same time) and minions means any attack against them is an attack against himself. I also probably wouldn't allow zone-wide blocks, but if I did the same rules and preventative measures apply. They CAN make selective multi-target attacks or blocks, but it requires splitting both the Weapon rating and roll between targets (something I'd definitely adjudicate as needing to be pre-split as part of the item definition).

But mostly I look at it as, how much of your refresh did you use to get up there? 10 point items require some combination of 5 enchanted item slots or 5 focus items. 5 uses requires 2 enchanted item slots or 4 focus item. That's a minimum of 3 refresh if its your ONLY enchanted item. For the same cost you can get Inhuman Strength and Claws for a permanent +4 to Fists as well as some nice side benefits that can't be taken away from them.

But if you tweak it the right way (and your GM lets you get away with it) you can break most mechanics.

The mechanical one - The Orbius spell used as basis for the silver chain bracelet is really wonky. I like the idea of an item causing a grapple, but how is it "administered"? Do you have to physically "hit" the target with the chain, and then trigger the items effect? If so, that could be the "create relevant aspect" needed to instigate a grapple. Orbius as written bypasses the action economy of grapples in a weird way.

My understanding of the reason Orbius is written the way it is, is written is that it is 3 shifts of effect per exchange. The initiating exchange, those 3 shifts are spend toward a maneuver (that could fail). The roll (in this case Weapons) is then used to initiate the actual grapple. On subsequent exchanges, the 3 shifts represent a skill substitution for the remainder of the effect (on resisting). In effect this isn't much different than a DoT or shield spell. But anyway, that's my take and I think it follows the Orbius spell's mechanics pretty well. I'll see if I can find some of these arguments you're talking about and see what the GM has to say, but its not worth starting an argument about.

The tactical one - He has a lot of zone-wide stuff. Does he have a way to not be affected himself by them? Or protecting his allies? At least the quartz and obsidian orbs seems like they're gonna target the zone he stands in decently often, and they might be as much an inconvenience to himself as his enemies.

Nope. But keep in mind that a zone is a relative thing. An average floor of a building is likely to contain multiple zones, so that's kind of a built-in drawback to the spells. Some, like the blasting rod, are decidedly "do not point this at your feet". Others, like the obsidian orb are very intentionally designed to work that way. I can't very well put an aspect on the scene NOBODY BUT ME CAN SEE; so they're all situational. That's what enchanted items are for. That's also why I'm looking to see if anyone has enchanted items/rotes/low-cost thaum spells that they think might be better or at least important additions to the toolbox.

Honestly I think Sponsored Magic is just a really bad way to model SoulFire based on the way the books portray it.

Two quick thoughts; take them as you see fit. One is that it seems far more likely that in the absence of any sort of evidence or witnesses the school would conclude that the clique had gone through some sort of infighting and trashed the bathroom in the process, but without those things couldn't do much more than detention. The other is that Lawbreaker stunt isn 't NECCESARY. Influencing minds isn't forbidden (veils, illusions, Harry's lust potion); only mucking with it on a fundamental level (like curing someone of their addiction). Or at least that's where the novels seem to draw the line.

Unrelated thing; your mind trick sounds like a one-hit takeout thaum spell... which are exceedingly expensive.

Everything still has to be phrased (mechanically) as an attack, block, maneuver, or counterspell, regardless of its narrative representation ('attack' with a summoned swarm of demon-wasps?  sounds nasty!).  I don't allow multiple aspects to be created with a single maneuver;  I don't allow durations not expressed in exchanges; I don't allow direct skill-replacement effects (note the 'direct'; invoke-for-effect is a powerful tool, but also one subject to additional case-by-case oversight);  etc..

It's not THAT ambiguous (IMO). From the RAW "In these cases, use what would have been the complexity of the thaumaturgic effect as a guideline for the power of the evocation." (Emphasis mine.)

I'll grant you there is a little wiggle room on if you use thaumaturgy or evocation's skills and focus items/specialties are not mentioned at all. As far as focus items/specialties go I'd have to say its probably not relevant since it doesn't come with specializations or focus items and it doesn't mention the Refinement power (or vice versa). But since "here's what it means" sounds an awful like "if and only if mentioned here", if you allow the Refinement power to work for Sponsored Magic and focus items aren't mentioned its either "they don't work on Evothaum" or "use the thaumaturgy ones".

Arguably (unless you want to get into the question of why weapons don't naturally break from hitting things during combat) you might require the attacker to take possession of the weapon before trying to break it. If you don't want to do that, then the player's dodge/block/etc and own consequences should probably come into play.

Interestingly satisfying complexity seems to be more about control then building power, even though we use Discipline (the control roll) to actually build the power.

Ways to gain shifts:
  Assistants - I'm a little surprised no one else mentioned this. They don't even have to be magically adept. I think its +2 per assistant per the RAW.
  Sacrifice of Power - There seems to be some implication in YS that participants can contribute power to the ritual (in a manner similar to the caster) by taking consequences without it inherently becoming Dark (or even Blood) Magic. Definitely not in the RAW though.
  Borrowing Power - This is mentioned in the RAW but the mechanics are very much left to GM fiat. I could see someone using the 'matrix' of an appropriately attuned enchanted item (already designed to contain a certain amount of power).

That said, building shifts isn't nearly as interesting as the question of keeping this from getting too 'broken'.
  Interference - Dresden's case files occur roughly 6 months apart, for instance, usually lasting between a week and a month. So lets say on average there's a 5 month lull between cases. But does that mean that Dresden has nothing going on during those months? We KNOW that's not true (from the short stories). I mean, sure, Dresden could skip that wedding. Of course setting aside the mundane concerns (pissing off the bride and groom), who knows what might happen if he does. Think your player(s) are getting a bit too complacent? Throw something at them.
  Worthwhileness - Technically since there's no time limit on setting up a ritual (particularly if you're making potentially long-term declarations). However, there are practical limits. What's the point of spending 5 months to set up a ward that lasts for 5 months? You'll end up having to spend that whole period to refresh it.
  Diminishing Returns - You can always house-rule things so that when you come up short (fail a roll) you've exhausted what that given skill's Declarations will do for you. Sure, you can spend some aspects to keep the gravy train rolling, but will it be worth it in the long run?
  One-Shot Workings - Since we're talking about Thaumaturgy in the context of wards, consider that you only get a free tag when the aspect is created. So, sure, you get a free bonus from that research you did the first time you cast the spell. And you can always spend fate points to invoke that aspect later, to represent its contribution but that tag's gone.So maybe that ward has a power of 20 for the first 6 months, but I but you can't make it do that again the same way.

Honestly, the maximum power of a ritual is basically dependent on the amount of time and participants you're willing to use. And limiting it abuses is just a function of using common sense and the fiction.