Basically my point was that they don't seem to be able to do literally whatever they want, just abuse existing mechanics to do things the mechanic shouldn't.
I really don't think we've seen enough of Carnymancy to say what it seems to be able to do. I think we can make a complete list of everything we've seen of Carnymancy, because it's a short list:
- Book 2, Page 21: KingWorld happens. Many people seem to think that was Carnymancy, but all we really know is that Charlie and Vanna were linked. Charlie might have contributed nothing more than Thinkamancy to the spell. It seems likely that he was doing the job of the Thinkamancer in that link, which suggests to me that he might be too busy to also contribute much Carnymancy. Even if there was Carnymancy in KingWorld, we have no way of telling how much of it is Carnymancy and how much is Turnamancy.
- Book 2, Text 44: Stanley thinks about the Arkenhammer: "Wanda said she thought it was a whole mess of different magicks, what'd she say...Shockmancy, obviously. Rhyme-o-mancy because it rocked out. Carnymancy because it made big flashes and sometimes could make things disappear (he never could do it on purpose, though)."
- Book 2, Page 63: Parson says, "Carnymancy. It's... the magic of rigging the game." Jojo confirms it.
- Book 2, Text 51: An arrow is deflected from hitting Sylvia. A delayed Carnymancy effect?
- Book 2, Page 66: Sizemore determines that the scroll is Carnymancy. Jojo said the scroll would send Parson home, but Jojo doesn't seem like a reliable source on that subject.
- Book 2, Page 83: Isaac examines the scroll and says that it might send Parson home, but that it also might destroy Parson. It's not clear whether he means that the magic is dangerous because it is chaotic, or whether he means that his understanding of Carnymancy is not deep enough to fully grasp the workings of the spell.
- Book 2, Page 86: Sylvia tells the story about her and Jojo. Jojo did something to save Sylvia, but it's not explicitly Carnymancy. Jojo "made a trade." Does that mean that Jojo used Carnymancy to do some sort of Fate trade, or is Jojo talking about making a deal with another caster for the magic needed to save Sylvia? I suspect that the magic really was Carnymancy because Sylvia certainly seemed like a rigged game to me.
- Book 2, Text 59: Parson reads the Carnymancy scroll. "Of course this was a spell. Of course it was. And it was built like...a key that would unlock the spell that had brought him here. He could see what it was. It was a spell to break a spell and snap it back. It would...fling him home. And he knew how to cast it."
Have I missed anything?