The problem with dowels is that the endgrain really contrasts. If that is what you want, or if you want to make a feature of it you could do it. Screw holes usualy aren't concentric, so it might be worth using a drill contersink bit that is preperatory to plugs, and install neat plugs. On the other hand plug holes are usualy 3/8", and that is cutting quite a bit of strength out of the riser.

There are people who can fill the hole, and then paint in the woodgrain, so you simply can't see that the holes were ever there. This is a furniture art. I don't know were you would find someone with those skills in your area. You could look under furniture repair, but it isn't a comon thing. You could also ask on a site like the Fine Woodworking site. If you do fill the holes with epoxy, the painting could probably be done later, so that if the bow became very valuable the repair could be made then.

You can patch those scratches with crazy glue. I use hot stuff's water thin glue, but any water thin crazy glue may work. Wad up a cloth about the size of a golf ball, so that one side is taught smooth fabric, add one drop of glue to the center, and imediately start lightly skimming back and forth over the surface. use enough speed and pressure so the tiny film of glue dries as soon as the cloth passes, don't get it so wet that the surface gets gummy and grabs the cloth. This produces a very nice, thin finish, instantly. Try it out on a similar test durface first.

In most cases values are higher if no repairs are ever undertaken. if you make these repairs you are destroying the artifact's value, to some extent, if it has any. The fact a bow is collectible to archers, doesn't mean it is important historicaly, as you know.