Aniline dye

Aniline dye that is water based is what I would use. Get a light color brown and apply it to your sap wood light areas. Assuming you have gotten all the old finish off, it will soak into the wood and start coloring the light areas.

You will not have reaction with any topcoat that you decide to later use on your stock as long as it is not water based.

Let the dye dry and if it is not dark enough apply another coat. Move some of the dye into your dark walnut and the color will blend so that you do not have a sharp edge.

You may have some feather (grain ends raising), just cut them off with some fine sandpaper.

One thing to consider, when you apply your finish, the wood will darken but it will be all over the stock.

The nice thing about dye is that it will high-light the figure in the wood and the figure will really bounce when moved back and forth in some light. If you use pigmented stain, it will muddy the surface so it doesn't put on quite the show.

As for a finish, I would go with an oil or a two-part car clear coat. What ever finish you decide on, you want it flexible enough that it will follow a dent into the wood and not leave a finish pop (white spot) if dented.

Hope I didn't confuse you.

Chris