So I sincerely hope he tries. He may indeed be a treacherous schemer, but he has no idea what a little Carnymancy can do to one’s best-laid plans.”
Dramatic irony much? I think at this point we can be fairly sure this is going to backfire spectacularly... The real question is how?
There is very little doubt that Charlie can easily overcome Bucky's Carnymancy, even at range, although he may need a linked caster as some sort of conduit. It might be possible that Charlie can use Carnymancy to dodge the contract itself, but being caught doing that would be an even worse blow to his reputation than switching sides in a conflict, since it reveals he's not really bound by contracts; his reputation is worth much more than a couple hundred thousand Shmuckers.
...But that's all just if he gets caught. The contract he's breaking is secret; neither Delkey nor Numloch know about it. All Delkey would think is that the horizontal strategy failed after all, since even with Dirtamancy bonuses the city fell to an air attack where its infantry were less useful. Dove and Digdoug could probably still escape to the Magic Kingdom even if the city goes neutral, since casters can bypass the portal off-turn, but Dove very well might be in on Charlie's ploy. She could probably just kill or capture Digdoug and then be escorted out by the archons as part of their agreement.
Still, it's a pretty big risk Charlie would be taking. I wouldn't do it in his place, but maybe he really likes his odds, or maybe he's got something else in mind.