The number of sinew layers depends on the thickness of each layer. Total thickness should not be more than 1/4" at the handle and not more than 1/8" at the tips. You can always build up as much sinew as you want, but I don't think the actual Hupa Indian bows had a lot of sinew on the back.
As for the pigment + glue + sinew for the final layer, I don't think it's a good idea, although it would provide a good base for painting. The reason I don't think it's a good idea is because hide glue (or any glue) becomes weaker when you add powdered pigments to it. If the glue is weaker, the last layer of sinew will not be doing much and may even begin to peel off as it dries or as the bow is bent. A better idea is to apply the sinew without adding the pigment and then paint over the top of the final layer of sinew with the glue/pigment mix.
As far as sprinkling the pigment powder over the wet glue, that's your call. It's very messy and some of the pigment will rub off after the glue is dry.
Oh yeah, tests have shown that normal exposure to smoke will not provide enough formaldehyde to preserve hide glue for 35,000 years.
