You may be asking too much from the buffing wheel and the Micro-Mesh. Buffing is for polishing a smooth surface, and not for sanding. The same is true for any Micro-Mesh above 4000.
There should be no visible scratches after using the 12,000 grit Micro-Mesh. The grit is close to 1-micron in size, and the only way you should be able to see the scratch pattern is with a bright light and magnification. Any visible scratches are there from prior grits; and they are often left over from the first and coarsest grit you used. You just couldn't see them because they were hidden in the dull surface.
The higher the gloss, the more you can see, and those scratches may not have been visible until the surface was made shiny with the Micro-Mesh. The best way to make sure the surface has no scratches is to carefully work the surface with the 400 and 600 grits before using the finer Micro-Mesh. The surface should be smooth and free of all scratches and blemishes before going past 600 grit. If it isn't you should go back and remove tham. You can improve the gloss after the 600-grit with 0000-steel wool to make sure.
Once you have a smooth surface at 600-grit, there is a choice between buffing or going to the higher Micro-Mesh geades. They are essentially the same particle sizes, and their effect on the surface should be similar.
Buffing after 12,000 MicroMesh can improve the gloss on some finishes and plastics, but White Diamond should never reduce the gloss from the 12,000 because they are to close to being the same.
