Who done it:
Someone from the merriment; no, too obedient and worshipful.
Someone from the swamp village; possible but unlikely. Doing so would either result in instant death upon consideration or fiery getting poisoned.
Family; Hard no
Enemy merriment; again possible, but they would do so on the orders of Candid, and he is letting her have a break instead of pressing the advantage
Third army; they don't have a steak in this fight
Daughter who got expelled; likely, not part of village when Hades made decree
Someone from the merriment (accidental); likely. if mistakes were made, she could have been accidentally poisoned
Because the numbness started in the nose and traveled away from there, likely source of poison was oral ingestion or contact with poison at the end of muzzle.

Intresting prematch food offering there. And given just how sneaky and intelligent that guy is, wouldnt expect anything to be found but unusual but totally passive compounds on weapon and body, but ingredients that can mix in teh body?

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Your faith in Azure Thrawn's abilities is impressive

I don't know how Fiery could have been poisoned but it won't really help Candid Diversion since a victory caused by interference by a third party doesn't really count. Also Fiery could die of this poison which wouldn't help Candid at all and I seriously doubt anykirin in either merriment is an expert in chemistry with enough skill to control the dosage.
Also if Fluffy learned about the details of the third army she would order Fiery to go with Rythil's suggestion which I suspect is part of the reason why she hasn't told her mother yet.