Yeah, I added the rust purely for color (the scales seem to have absorbed a good deal of iron oxide to get their color, I figured adding a little more would help me match -- it kinda worked).
I'm not as clear on the epoxy, though resin-based epoxies should at least have the potential to last a very long time. Cyanoacrylates I specifically researched. Museums use them, so I figured it was probably good enough for me. CA/horn slurry is incredibly strong when it cures. Stronger than either one by itself, and unlike straight CA, it's also relatively ductile.
I haven't done a lot of experimentation with the glue/horn mixture, I just figured out a workable process and I've used it ever since. There is probably a eutectic ratio, but the way I do it doesn't have much room for experimentation.
What I do is this: I pour the horn dust onto the surface and pack it down into the crevices then carefully drip Rockler's 'thin' formulation CA onto it. It wicks almost instantly into the mass and cures almost as fast. To get that finish even, I sand it down flat then do another coat or two of CA. It helps to scratch up the surface around the edges so that the glue feathers into its transition instead of leaving a clean boundary (extra help is scratches that follow the grain). But the glue/horn hybrid is so tough sometimes I have to use a grinding wheel or the belt sander to get off the excess.