Yes, whether the encounter leads to combat can vary and but in my mind if its randomly generated with the whims of the PCs and the Reaction Die I still treat it as potential combat; plus I wasn’t opposed to them on the ground of too many combat encounters, just too many encounters in general occurring each exact hour.
Yeah the Gulch does lack description on what makes it arduous terrain, particularly since it as you pointed out is titular! It would’ve been great to see what you wrote for it, perhaps an expansion later?
When I was looking at the Shadowdark corebook and considering overland travel I was considering the gulch as difficult terrain (which multiplies the time by 2) chiefly because arduous would make it a flat 8 hours regardless of walking or mounted, which I take as reserved specifically for travelling through mountainous terrain.
I think the shadow encounter should be described as occurring at sunset instead of during a specific numbered hour, that way it accounts for PCs who set off on their journey early/late, encounter time, and/or pausing to rest along the way.