I can't address your shader tree since I use RayDream, but I might be able to shed some light on your white line around the "white is invisible" issue. A while ago I was playing with transparency maps and had the same problem. It turned out that it arose because I'd made my maps as JPEG's, and the compression scheme caused the white around the decal to be a little off white. My solution was to save the image as a BMP, but if you're on the Mac, select your favorite uncompressed format. I put the decal on the object and woo hoo! it worked perfectly. I hope this helps. Does Carrara have the paint shapes that RDS has?