RE: Terrain boundary editing.

The triangles look to be correct in that they are going to the drape void where it is inside the terrain - as it is a void, there is nothing that can be done outside the terrain. The boundary is showing the extent of the terrain, and there should be a void at the boundary. To me, this is correct.
If you wanted the boundary to go around the void, I think you would need to add a breakline outside of the void, which is making up data.