Yeah, zippers. . . . They add weight, bulk and decrease warmth. Yet, they make a bag more flexible in terms of temperature range. Someday, I'm going to take the quilt plunge and see how that works for me. At home, I sleep under a quilt (in a cover) only and I seem to adjust my coverage without waking, so maybe it will go well.
Yes, butane needs to be kept above freezing although the heat capacity of a canister is so little, that you also want some reflected heat to keep it going once lit below 45-50F ambient. Propane-butane is better and more propane is better yet.
Camera batteries? Not so much anymore. NiCads - yes, need to kept warm, but I haven't had a NiCad in anything for a decade now. Lithium batteries are quite good in cold temps, down to -20F. For cameras, I manage the camera's temp to minimize condensation on/in the camera and use a ziplock bag to keep condensation from forming on it when the camera is cold and the surroundings humid.