The matter of the unbroken 1:6 side slope was solved by a review of the parametric constraints wherein that slope was set.
I've created a new post requesting that parametric constraints that set a slope should be indicated on the dynamic cross sections. communities.bentley.com/.../127221
Nevertheless, I continue to have the strange problem where when I set the clear zone to be 30' or more, this end condition has no solution. If it's as high as 29.9', it has a good solution.
Edit: This, too, is solved now. It turned out that locating the minimum depth of fill and the clear zone offset were at exactly the same spot, and the minimum depth of fill had been designed to indicate the direction to seek the existing ground.