I'm going to do a quick field check on the area and get some video / pics. The noise you noted at the stream crossing is just that, I'm sure, but I don't know what caused it, so I want to do a quick field check, if only for that.
On the issue of generating surfaces, there is an easy way to constrain it somewhat. Specifically, we can create a "corridor" that parallels the horizontal alignment and is offset several feet on either side. It should be simple to do in Sketcher. Basically, roads in the US have a 40' -60' right-of-way from the centerline of the road. Knowing this in advance, we can know that points which exceed that limit do not need the surface computed.
Anyway, I've got a couple other things in mind that I'm working through. I'll post something later... in the other thread.