Lara (in Turn Coat) explicitly denounces the abandon with which some members of her Court feed (though I can't, for the moment, recall the name of the specific WCV that inspired her comment: the female that showed up at Zero).  I have no doubt that she would be accepting of a certain amount of restraint being enforced on their ilk.

My thinking was that the White Court, as a whole, do not see themselves as humans who have a hunger.  They see themselves as predators and us as food.  They would see any attempt to get them to eat less as an aggressive move on the part of the Council.  And like Richard said, they would go absolutely spare if a wizard were trying to study them intimately enough to to affect their hunger.  That's only a few steps from being able to neuter them and with their control fetish they could never let that happen.

If there were enough like Thomas who were willing to take on the White Court in the same way as was done to the Red, then you might have cover and subjects enough to get something similar made.  Maybe.  The whites aren't the reds though; control and power are like crack to them.  I just don't see it happening unless you're willing to radically alter your world enough to get a civil war within the White Court going.

Also, did I see the Buzzard imply that teleportation spells were simple above?

Harry managed a short distance teleport potion in the first book, granted with the help of Bob for a significant lore boost.  Which means he could pull off the same as a proper thaumaturgy spell.  Also stands to reason that LoS and extremely short distance would be somewhat easier than non-LoS and a couple dozen yards.  Exact amounts would be up to the GM.  Simple, no, but doable at 8-10 base refresh.

Sounds to me like you are trying to create Geo-Alchemy. Big Earth magics, mixed with trasmorgrative spells, to let shifts in form be permanent. With the addition of polish and cut to the jem, I would put this about as difficult as building a sparrow from scratch. Not impossible, but not a snap.

"Also, consider that the consequences and other aspects in those situations are often the result of the spell, but they aren't sustained by the spell.  Attacks of that kind don't really have an intrinsic duration--they just happen, and they're done."
(YS266)

I think there's a good argument that it's not even transformative since it's still the exact same number and type of atoms;  only their positioning is changed and carbon is just as happy being diamonds as coal.  I don't think you'd need to allocate even one shift to duration.

Yeah, it'd definitely take more complexity to arrange them in a pretty fashion but that's not really necessary if you can just pay a jeweler to do it for you or tell them lop off enough to cover the cost and keep it.

the one canon example we have, she tried pretty hard to get herself killed during the spell (though that could have been about the convenience of making sure the body she left didn't get uppity).

Stat it as Kemmlerian Necromancy (YS291).  That way, unless they actually are a Kemmlerite or have access to his teachings, they have to do it as thaumaturgy which it absolutely is.  Beyond that, it's up to the player and GM to determine the amount of complexity.

Bear in mind though that the Wardens will be coming to ask some pointed questions about violating the 1st, 2nd, 3rd, and/or 5th laws.  Not to mention their fervor relating to all things Kemmler.  It's extremely likely that it most certainly will be a death curse, even if you pull it off.

A simple rearranging of carbon atoms in a proper crystal matrix? Using what elemental magic? You realize we don't even know what an atom looks like, but can only hypothesize what they look like.

Well, I was thinking thaumaturgy but it could fit under either earth because they're both rocks or spirit because of the teleportation method.  Remember, we do have precedent for taking something apart at the atomic level and putting it back together again with only Harry's skill in SF.

AS for knowing what carbon atoms look like, we do.  Granted it's not exactly a good picture but with magic the point isn't so much correctly knowing what a carbon atom looks like as much as it is believing you know what you're doing and that you should be doing it.  A little time spent with a chemistry textbook can take care of the former;  the latter is up to you.

Given the troll's collapse into minor trolls / parts and Toot-toot's growth, I'd say there's a possibility that fae don't die on earth so much as (semi) permanently become "less" when "killed".  Not sure that follows in all the fae though...seem to remember the

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Fomor actually dying

but am not certain. 
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I still tend to think anything physical, including ectoplasm, has the potential of crossing a threshold.  The caveat is they'll need to be strong enough to maintain physical existence in spite of losing access to metaphysical abilities.  Or at least having said metaphysical abilities reduced by the threshold's power. 

For examples, look at everything from the demon in Storm Front to various fae (Jenny & Lea at least), to

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wraith's

in GS.  Also, GS seems to make it clear that some thresholds may

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recognize and allow allies.  (Harry entered the church without an invite successfully.)

In the book, he wasn't all that attractive, although he definitely looked human.  He talked a good game, and had a certain amount of charisma, but relied on threats and mind control to get the girls.

Threats and mind control, booze and flattery... whatever works.