Hi Nuutia,
You could also try changing the OpacityMaskClipValue on your landscape material. That will cause the point of cutoff to move around. Also painting your mask value a value a bit brighter than black will have the same effect. The default clipmaskvalue is 0.333 so try values around 0.1 to 0.25 for your mask brush color and see how much that helps you.
The corner you talk about is because the values are interpolated using the vertices. Since all the triangles are running in the same direction, you will see interpolation go this way on some corners.
Having some sort of foundation type mesh will help as will making the walls of your building a bit thicker.