I'm an idiot for not suspecting it sooner. It was me all along!
Among the other usual junk I forgot to strip off of prisoners, I found a cobalt pick next to the inside of the door to the Thief Warmer. And since I'm pretty sure invaders don't typically arm themselves with digging tools, it was probably Rafeb that went in there to turn it on. (I found a few bodies in the mines so his the only one still unaccounted for.) And then he got stuck in the smoke and couldn't path out before bleeding to death while on fire, the duke took the fall for whatever reason but since I've blocked off the door nobody can get in there to identify the pool of grease that remains of Rafeb. Whoops!
Though now I can't help but imagine Duke Kixiobin being confronted by Rafeb about how he can remain so calm and continue to gain weight while everyone else is panicking about having enough to live.
---
"Where is it, Ad? Where's your secret supply?"
"I don't know what you're talking about friend."
"Don't play dumb with me man! You used to be the only one I could trust! I thought you trusted me!"
"I do trust you, Rafeb. In fact I trust you with so much I'm giving you an extra special mission to go light up the chimney-stack. And should you find anything- interesting- while you're up there, well I can't imagine there'd be more than enough for two very close friends, you understand?"
"... Thank you, Ad. I- I'm sorry I-"
"Nothing to worry about friend, just remember to close the door carefully so you don't- spoil the surprise. Oh, and you'll need a concrete excuse to go to such an out of the way location without arising any suspicions; here, I've filed a request for the pyre to be lit at midnight tonight so the caravans can find us through the blizzards, don't be late!"
"Oh I won't, you clever scoundrel, you."
"You would know one, traitor. Have a nice day."
---
"Ah yes, you're the hauler, I trust? Jonu is in sore need of a proper offering shrine, so I was hoping you could take this masterful wooden hourglass up to the great stone spire, there. Great, be sure to have it done at least two hours before midnight, so the priests can rehearse the ceremony before it's time. Fantastic."