I don't understand, why do you think we need to perfect steam engine as a prerequisite to cannons? I support doing that anyway for completeness reasons, but the only part of the steam engine we need for the cannon is the boiler, which is already working perfectly. Everything else is cannon specific problems which don't benefit much from the work we will do on engines, which is mostly turbines.
I don't think we are 2 designs and 2 revisions to get the steam engine useful, more like one design and one revision. Last thing missing to have it be better than rowers is better sizing of the turbine.
Steam rifles are not a serious proposal. they are a far future dream, but they require so many innovations that we can't hope to do anything useful with that idea in the short or mid term.
edit: about the fireball cannon, I owuld like to know exactly the mechanism in which fireball operates. gunpowder works because its combustion turns a solid into a lot of hot gas, icnreasing pressure quickly and enormously. Depending on the working of the fireball, it may not be able to produce much thrust even when confined. It incinerates enemy squads, but you just need heat for that not an explosion. (note: heat would still expand the air and provide some pressure.). Of course, I am not saying it won't work, but we should ask the GM if the fireball is actually explosive or if it is just a ball of hot gas that spreads on impact or something.
In case it does properly explode, fireball cannon could work I guess.