Good point about flue temperature. No doubt it affects the draft. Based on what I've seen in antique coal stoves, they didn't necessarily use a larger flue; rather, they used a much longer one. The long flue would hang just below the ceiling of a building (think about a one-room schoolhouse or whatever) and radiate heat all the way along.

The longer flue has the same effect: it creates a vigorous draft, even when the fuel source is getting low.

The other draft factor is "over-fire air." Many coal stoves had mechanical thermostats that would admit air above the fire to maintain the temperature output.

And sometimes, as you have noted, a forced-draft fan was used on the intake. That was a late innovation. (However, in modern times, if I have electricity I won't be pulling out the coal stove.)

BTW: The coal we have here (Alberta, Canada) is very low in sulfur. Unlike our oil - go figure. Personally, I don't mind the smell, but the first ten years of my life were in a coal-heated house. So for me it brings back pleasant memories. But as they say, YMMV.