Hello Andrea, and welcome from a fellow European!

The question about value I would gladly leave to others here who are much more knowledgeable.

The question about the darkened Micarta I can answer as a knife maker of 42 years. You can only remove the darker color by removing some of the material, and it depends on the Micarta how deep the darkening has gone. In my experience it can easily be 2 mm. I don't know if that would be desirable, to me it would not be. What's more: you cannot keep the material from darkening, because it darkens because of UV light.

Personally I would expose the lighter part of the handle to sunlight, so the lighter part becomes as dark as the darker part. My 2 cents.

A beautiful Sportsman's Bowie by the way.