I don't know how high zeppelins can go as a practical matter. Theoretically, with no payload at all they should be able to reach a height where the air outside matches the density of the gas inside, so depending on payload mass the actual height will be less. From what I remember Safehold is slightly smaller than Earth. That means air density would fall off more rapidly with altitude. You'd have to run some specific numbers to know if the effect was significant or not.
Believe me, I knew. The zeppelins of first years of World War I could climb up to the 2000 meters, and temporarely - by using enignes and altitude control surfaces - up to 3000-3500.
At the heights they've historically flown though, massed rifle fire from the ground could punch enough holes to cause noticeable leakage (and force them down).
Historically, there was no cases when airship was brought down by rifle fire. Especially by the fire of mid-XIX century rifles.
bullets passing through canvas, mylar, or whatever the gas bag is made of, probably wouldn't spark but I imagine there's still a remote chance of it with each hit.
No, they would not. Even the tracer bullets by themselves would not have a significant chance to ignite the airship. Only the combination of HE and tracer bullets was deadly to them; the HE's ripped the envelope and formed a huge gas leaks, and the traces ignited the resulting hydrogen-air mix.
Father Paityr isn't going to convince anyone flying machines are within the bounds of the Proscriptions, no matter what creative BS he comes up with to support the idea.
Please! The hot air is rising, because the God created it so. There are another gases, that are lighter than air - of course, because God created them so! So, what's wrong with the idea to use the gases to fill the ship and float in air? Flying? Be serious; only Archangels could really fly! Whaty are we talking is a mere floating in air, as ships float in water. And the ships are allowed, of course!