That's what I mean about mortals. A single wizard among them could have far more easily recognized what they were facing (high Lore), used Thaumaturgy to ward the group against Outsiders both physically and mentally and his evocation would have been stronger than explosives. Not to mention he could sense that the "cute girl" was an evil cultlist by soulgazing her.

You're applying Dresden Files character types to Cthulhu. Which really doesn't work. Cthulhu is intended to be pure mortals struggling against overwhelming supernatural creatures. Cthulhu magic tends to destroy the caster's sanity when they cast it. If you translated a Dresden Files power wizard to Cthulhu terms, the character would be completely insane, and probably doing far nastier things for power than the nastiest bad guys in the Dresden Files ever have.

My apologies, yes the stress rule is tucked away in the text - it really should have something to highlight it a little better, it's quite an important rule. However, as I read it potions don't get the stress spend option anyway.

As far as I recall it's perfectly legitimate to declare an aspect on an NPC. But it's reasonable for the GM to refuse the declaration if they feel it necessary to do so.

As far as I can tell from the rules, it's a little less flexible than it appears, but is quite acceptable - indeed one of my players is effectively playing a Crafter, and it's absolutely within the spirit of a Focused Practitioner type. But I'd disagree that a Crafter would be a bad-ass Spell Slinger when compared to someone with both Evocation and Thaumaturgy - after all they will have an almost identical set of High skills to the Crafter (Discipline, Conviction and Lore), so will be able to perform exactly the same potion-style abilities, and then keep going using their Evocation.

Balance-wise:

If you use slots for Enchanted Items then they have the same effect every time. And while you can change them pretty much when you want, the process for creating an Enchanted Item is described as similar to that of creating a Focus - which takes months. So there's no chopping and changing from session to session with enchanted items.

For Potions, yes you can leave the slots open and simply spend a fate point, or make a Lore roll to have the appropriate one. But potions have 1 use only, unless you're spending slots to get extra doses of the same type of potion. The effect strength is also limited to your Lore, unless you spend multiple item slots or invoke aspects. So basically you've got a fairly flexible degree of spell effects, but for a very limited number of uses. Someone with Evocation, even a beginning character can equal or beat that - and with the bonus that if they are captured they can't be simply stripped of their ability. As a further point against the Crafter, there must legitimately have been time for them to have created the potion earlier - where Evocation needs no such justification.

It's in the Your Story book for the rpg, not in one of the novels.

To be fair bothe questions are possibly a bit of a moot point. Firstly I'd argue that there's probably nothing that can't be countered by magic in some form or another. If psionic power is just a focus of will, why shouldn't counter magic, which is effectively also a focus of will, work against it? You can colour it how you like, whether that's the wizard smashing his will into the psionic's will, or into the power of the mental construct. If the psionic power is causing a physical effect then it can be countered like any other physical effect.

And secondly just because you don't believe in magic, doesn't mean that it doesn't believe in you... Sanya is after-all an agnosic Knight of the Cross (admittedly it's a slightly different case, but I think still valid as an illustration).

That's not to say it isn't an intriguing concept - it is - and that a number of supernatural factions wouldn't be interested in such a talent - they would, even if it's just to disect it and see what makes it tick. But I do think its one you've really got to think through with your GM to work out any problems ahead of time.

Possibly was just rolling lucky. Without invoking aspects someone with Superb as their skill level should only be breaking out of a block like that half the time (that's not taking into account supernatural powers). Superb skill level equating to "a veteran or masterful capacity, or the combination of extreme talent and good training."

It may be that your GM is setting the NPC abilities too high for your group - it's easy to do when you're getting used to the system. I've done it myself. Or you may just have been up against a particularly skilled opponent. NB The GM should be stating any aspects they are invoking in the same way that a player has to.