“ A couple of things I'd welcome input on - when doing this with heat is the durability of the finish any different than doing it with nitre salts? Similarly, what is the overall durability of the finish? I realize it polishes off easily. Does the finish bring any additional rust protection? Any input is appreciated.”

The coloring is not durable.

There’s no difference in the colorings durability done with either salts or flame. It’s still coloring done by heat. Salts just heat up and surround the steel in a constant temp bath. Whether you accomplish it by using a flame or salts, all your doing with either method is heating the steel to bring out a particular color whether that be straw/yellow, purple to various shades of blue to black and even gray.

It provides no real rust protection because the coloring doesn’t hold oils like blacking does.

One trade trick when using a flame is to heat the pin or screw from the bottom up. It allows the pin/screw to heat more uniformly. Don’t apply the flame directly to the steel, move the steel in and out of the flame and go slowly.
The higher the polish, the better the color.