Quad face tools challenges

After you have conformed (similar in form) the new terrain over the existing ones, you no longer need quads.
After you drape the road over the terrain, it’s normal for there to be no more quads, and you don’t even need them to move on.

In the first clip, I recommended the method with quads both for speed and because you wanted to learn more.

In the second clip, you can see that you get the same result with native SketchUp tools.

The idea is to get a surface with more faces/vertices to be able to capture more details from the survey surface.