I tried adding pavement, but i have exceeded the amount of terrain layers, if i add one more i get the colorful rainbow textures. But i think i will remove the grass and add some cobbles, i can't guarantee it will look good as im not good in making nice looking terrain.
And regarding that small hill, i checked and there is some plants that might have been with collision, i turned it off.
I could be wrong on my solution, though it has worked for me. This is because of multiple terrain materials being visible on the same quadrant. ie you have painted on several layers in the same spot, you can change this and make it work assuming you don't need all materials to appear in that spot, it isn't so much the amount of TMats as the amount of textures and instructions for the TMats on that quadrant. This is why the "rainbow" only appears in quadrants not the entire map.