Something else to consider, it's very possible, indeed, probably even the most likely scenario that, given the fact that there probably aren't any trees left in the Caldera or East Valley (we were just told last update that it's all been occupied and used by the orc ecosystem, and probably has been for millenia), that the greenskin buildings aren't made from wood at all, as we've been assuming, but that they're literally mud huts, made of wet adobe. If there's one things the orcs wouldn't be lacking, it's dung to mix the earth with. Any wood would long since have been rotted away by orcoid fungus, and with no source of replacements, that doesn't leave much.
That would really stymie plans to burn it down.
Well for one it wouldn't just me Mathilde starting the fires, the vote outright has a section about other parts of the army using their own methods to add more fire to the mix such as the Rangers and probably Halflings.
Secondly the main difference is that when a random fire gets started there is usually a bunch of other Greenskins nearby to try to put it out or there is only one fire at a time. Without our method we would be starting multiple fires in a town that has had large sections pretty much abandoned, meaning that the fires will have time to grow beyond a critical mass before the Greenskins even have a chance to try to stop them.
Thirdly a deliberately triggered fire is going to be engineered to go out of control as fast as possible, not to mention an accelerants we decide to use.
Mathilde is incapable of carrying meaningful volumes of accelerants. And I very strongly doubt that the greenskins are capable of organising a fire brigade
And I also addressed the very real problems with getting the Rangers involved in this. They're dwarves. They don't run very fast. Lighting a literal beacon to mark your location is begging for some orcs that fancy a brawl to come to look what's up and swamp them in bodies.
They don't have to literally run away to cause problems. They could do any of a hundred ways of demanding more pay, saying that the assault is clearly over, or simply following the caravan back. Their commander was also hesitant in confirming their loyalty unless they attacked all but immediately off the party high. Also if Belegar wants this to succeed long term, avoiding a reputation for being a paymaster who will screw you out of a paycheck unless you do overtime isn't the way to go.
The success criteria of the expedition weren't defined when the mercenaries were hired, so we aren't breaking any promise or deal. We only decided where to attack after we arrived, so whether or not the Citadel was included in them can't have been defined to them earlier.
This campaign has also been astonishingly fast and low casualty. These mercenaries came expecting to besiege old dwarf holds. Compared to what they signed up for this has been a walk in the park. In other circumstances they could still be weeks away from taking their first peak, rather than having a speed run of victories.