Let's say we have Harry sitting in an empty white room... with the Giant Jar of Pickles.  Harry's really craving a pickle, but the lid is stuck beyond the power of mortal muscle power to budge.  Harry summons up his will and begins casting powerful but subtle force effects to twist off the obstacle to sour salty goodness.

If he keeps the spellcasting up, he will eventually be taken out, no matter how many consequences he takes.  What sort of 'taken out' result would be appropriate in this admittedly rather silly scenario?

The jar of pickles has no interests or preferences.  Yeah, the GM certainly does... but there's no context to guide the probable outcome.  If he burns out his ability to use magic for a while... how long should that last?

That Mort didn't exist when the RPG was written.

  As I mentioned in the very first post.  Nevertheless, the fact that the novels have expanded our understanding of the Dresdenverse means that the RPG is no longer adequate for producing the 'feeling' of the novel's setting.

And he can easily be modeled by giving him "Mimic Abilities" coupled with "Human Form" (to limit the use of powers to times when he is linked with a ghost) and a list of ghosts he might have access to.

  That might be an adequate representation of a single power; he has others not readily represented by the rules.

Well, they are taken out of the conflict. Regardless of who did it, there is a winner of the conflict, and that's their opponent, so he can dictate what happens.

  There doesn't need to be any conflict.  A wizard all by their lonesome in an empty room can cast enough spells to overflow his stress boxes.  Nor does there need to be an opponent.

It makes perfect sense that when there's an opponent, and they inflict the stress, they decide what the result is.  When the character is inflicting the stress on themselves... do they decide?

When a character is taken out by stress, their opponent decides what the consequences of this will be, moderated by the implied context of the contest.  Seems reasonable enough - a truly unfriendly combat could end with the character dying, while a relatively restrained arm-wrestling conflict might result in bragging rights but no lasting physical consequences.

So how should the mental stress associated with spellcasting be interpreted?  There's no opponent character involved, so no character-desire context.  It's just the GM's opinion.  Which wouldn't be so bad, except... The book describes mental consequences as extremely severe.  I suspect they're talking about the worst possible cases, rather like dying is the worst possible outcome of a physical struggle.

The player can't concede as an action - there's no struggle, and therefore no action other than their own that would push their stress over the limit.  So what happens if someone goes over their own mental limit?

Nowhere in the novels does it say that you need to learn spells. Magic is you imposing your will on reality, and that's covered pretty well by the game system. I'm sorry if you don't like it, but it actually does a pretty good job at covering the dresdenverse as we know it (or at least knew it up to that point).

Overall, I'd say your last claim is true.  But on this particular issue, the rules do NOT do a good job of representing the novel's version of Dresden's world - not up to the point of Small Favor, and definitely not after that point.  There are other points where the rules also break down - such as Spirit evocation being so much more option-heavy and thus more powerful than the other elemental evocations.

I've seen the suggestions for specialist mages using Sponsored Magic with abstract concepts, somewhat like the way the game handles big-N Necromancers.  But I never got the impression that Mortimer was a servant of ghostly powers, just a guy who cared.

Even under bog-standard canon rules, Mort's a better ectomancer than Harry. His skills might be a point or two lower since Harry's a much higher-Refresh character, but unlike Harry Mort has relevant focus bonuses.

Oh?  What focus items does Mort have?

Is there a particular character you can't seem to build in a way you want? Maybe that'd be a better way to solve your problem than trying to argue in the abstract.

A magical practitioner who doesn't have the breadth of a wizard - and who can't do anything at all outside of a very narrow theme - but who has an advantage within his specialization.  Who is better at what he does than any wizard of roughly comparable experience and training.

I don't know - I suppose the right stunt could simulate this, in the same way that stunts are used to represent how (the book's example) a heart surgeon is distinct from a doctor, or a doctor distinct from someone who merely has lots of medical knowledge.  But heart surgeons can still do generic surgeon-y things.  They are limited in their mastery, not nearly as much in their general ability.

I want to be able to make a thematically-limited mage that isn't hamstrung within that limit, but empowered.  With the existing rules, even with the restrictions taken out and ignored, that isn't possible.  Any [something]-mancer, no matter how experienced, is going to be inferior within that field to a wizard who is experienced with the subject magics.