Currently the RadSlider doesn't support the first scenario and the only work around is to construct a new LinearGradientBrush on every AverageGrade change.
The easiest solution for the second scenario is to edit the RadSlider template and more specifically the HorizontalRangeSelectionThumbTemplate in Blend.
I've attached you a project with a sample work around for the first scenario and an updated template for the second, so could you please examine it and if you have further questions feel free to ask?
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