there is absolutely nothing in the rules that forbids creating a thaum. ritual that replaces your rapport skill with a magical skill of 9 (or 13, or whatever) until the next sunrise. RAW, that's completely valid.
So is a spell that replaces your guns with a 9 for the same duration.
This is pretty much directly out of the Thaum. section of YS, and is allowed. The complexity of the ritual is equal to the level you want to set the skill. All thaumaturgy ends, unless you intentionally change duration by varying complexity, at the next sunrise.
So every morning any thaumaturgist could cook up a simple little ritual that gives him 5 alertness. (or any other skill).
Superior Senses (Biomancy)
Complexity 5
The character uses magic to heighten his own senses, fine-tuning them to perfection. This lasts until the next sunrise.
If you're a high discipline person, you could easily dump 3 points of power into this spell every exchange and have it completed in 2 exchanges. As the exchange time for thaumaturgy is NOT listed in the books but IS supposed to be longer than that of a combat round (as that is evocation's speed), I'd argue that any thaumaturge worth his salt could pull this off in 2 minutes.
Because this quickly turns into an alchemist with lore 5, item strength +5 (from foci and specializations), so the person now has access to on-the-fly spells that give them a rating of 10 in any skill they want, I -as a GM - began imposing a consequence for any spell that set your attribute over your normal maximum skill level for your refresh level. I also ruled that all potions last at most one scene, and the default potion time is "a few moments". Which is more generous than the novels, where potions tend to last what would be just a few combat rounds/exchanges.
An example of this from our game was:
Eagle Eye Potion, take 1 (potion strength 10)
Drinking this potion gives the character guns 10 for a few moments. After the potion wears off, the imbiber takes a mild consequence of "blurry vision".
Eagle Eye Potion, take 2 (potion strength 10)
Drinking this potion gives the character guns 5 and alertness 5 for a few moments. No consequence is applied (neither skill went over the normal maximum of 5 for that character, he just made the potion effectively carry 2 spell effects, which is allowable)
---- Anyways, yes, you can totally use a spell to raise your rating in a skill to a level equal to the complexity of your spell. Whether or not you can do this on the fly with soulfire (thaum at the speed of evocation) is up to your GM, as that issue hasnt been clarified (soulfire doesnt specifically say "at the speed of evocation", where every other sponsored magic says that. It has not been clarified if this was simply forgotten, or was not supposed to apply to soulfire... so it's debatable).
If you CAN use it on the fly, then you could model the character's CALM DOWN! action as raising your intimidation skill to the level of power you set the spell at, then making a normal intimidation attack. That attack would deal normal stress and cause consequences but would NOT specifically inhibit actions as you stated. However, if someone got an aspect of "cowed" or the like, you could then, yes, invoke for effect (tagging the consequence for free, as you can do once with any consequence that you create), meaning that the bad guy either can't act but gains a Fate point, or can act but pays a fate point.
