One other factor that hasn't been mentioned - If you were able to make a perfect rolling bevel and eliminate the exterior gap, you may have to modify the Pygmy constuction method. The Pygmy method, which is different than other stitch and glue designs I have seen, involves applying a bead of epoxy to the outside seam then removing the wires prior to glassing. That small bead in the exterior seam is what is holding the boat together while you glass the exterior. If there was no gap at all you may have a problem getting enough epoxy in to hold the panels together.

One other point - I think most of the sloppiness of the exterior seam probably comes from being overly aggressive in sanding the chines round. If you are not careful it is fairly easy to locally cut through the first ply of the plywood, creating a jagged looking seam. I speak from experience here! If you align your panels edges very carefully and are cautious with your sanding, you should produce a nice neat contrasting line. I personally think it looks kind of nice, but if it offends you I would try some of the other suggestions about lightening the color of the epoxy before I would try to but an accurate bevel on a 17 foot long piece of 4 mm plywood.

Good luck, Ralph