Sometimes the Sliver or Max Triangle Length settings are insufficient to properly limit the formation of "web" triangles that form between to areas that protrude from the survey. I tried a few suggestions to turn off survey rules but these are not working. I know that you can define a survey to create a terrain automatically or you can create one after the fact. Ideally, this type of edit might be needed for either workflow.
Yeah editing a survey terrain isn't very good, especially compared to Civil3d. They are locked down somewhat as you have found. Your best bet would be to create a shape and add it as a boundary.