No, there are no dark edges when I paint a merged texture across multiple surfaces.
Yes, well this is what I was curious about. The images you have do not fit the surfaces you made. In your version you stretched the image in one direction to make it fill the surface, but this makes the dimensions depicted in the satellite image wrong I would think. You could resize the surfaces to fit the texture. If you know the surfaces are the correct size and you stretch the image to fit then you will have to know that measurements taken from objects in the image are wrong.