Did I? Can’t find that offhand, but regardless of what I said, my intended meaning was a Theurge-crafted reservoir. Not in a super blood-expensive permanent-enhanced-telos way, like the Plektoi spears; just drawing out the metal into the right shape, without seams or uneven thickness that might lead to failure under pressure. The kind of Theurgic craftsmanship that also allows them to make beautiful objects or elaborate distillation equipment out of glass.
Once the Theurge in the manufactory has made a high-quality compressed air reservoir, the rest of the rifle is assembled by hand, and the reservoir can be filled in around 10-15 minutes by hand-pumping, allowing around 20 shots before a refill is needed. (If you’re in a position to get off all 20 shots against a Theurge, you’ve got excellent enough cover that you might even be able to refill; although if you haven’t brought the Theurge down in 20 shots, you’re probably screwed regardless.) More like a Girardoni than a modern air-bottle airgun.
I’m really not sure about that. Canal and mine pumps, sure – but the pneumatic engines are a big enough tech step up from rifles that I don’t think they need to exist in the factories. (Though they might work well as a G5 tech advance for a high-INT player trying to diversify out of Theurgy as many ways as possible.) And explosive mining in the gameworld uses miner’s powder – since there’s generally no need to fear a Theurge spotting it and attacking when used for that purpose.
You’re absolutely right that the well-established alchemy of the world fits well with chemical solutions to problems, and I like the suggestion of chemically inert substances that are only explosive when mixed. Let’s say those new, far more devastating shells were invented, tested, and rolled out against Halassur along with human wave attacks forty years ago, as part of the horrific surge that the Halassurqs remember as the War of the Secondborn.
Since shells can only be borne to within target range in Halassur by ship or in limited quantity with flying Theurges, this would have led to a shift in Halassurq defensive resources to focus even more intensively than before on naval interdiction, magically enhanced ballistae in coastal forts, and high altitude air patrols.
Answering a question that open-ended would take me away from writing for too long. Sorry. 
Sorry, I was totally unclear there. I was imagining two possible games, “Choice of the More-or-Less-Peacefully Disintegrated Empire” or “Choice of the Militarily Defeated but Not High-Anarchy Empire” – not as a single game where you could pick a playstyle somehow modeled on those two historical examples. I can’t imagine what a single “Gaidar or Yoshida” game would look like either.