Looking at the UVW-unwrap, it might be worth reducing the shape steps in your loft parameters to remove redundant geometry.
I am not sure if asm tweaking removes the geometry "underneath" (I suspect it doesn't) but this is a large number of redundant polygons that you could consider removing.
With regard to MipMapping, removing the Mips will force the highest resolution Mip to be used. This works pretty well for ground textures but in certain circumstances will create shimmer and/or moire effects. It does a pretty good job at getting rid of the mapping errors (white lines) with lower Mips, and means that users don't need anisotropic filtering.
1. Removing mip maps has not always resolved the issue of white visible lines in FS.
At times, I've been forced to map the textures using a pixel or two of edge margin. To avoid any discontinuity, I've resampled the textures to contain a redundant edge pixel, but you may not need to do this. It may be unnoticeable if a single pixel row or column is missing.
2. The steps in which you instruct the reader to add black to a layer and later white to a layer is not easy to follow. A simple image of your layers would make it completely understood with no additional words.