My solution suggestion would be to have two "value source" types, one driving the UI (e.g. blue border (Local value) in Styles for properties that have <Setter/>, default/gray border for set dataset properties, etc...)
and then one driving the logic, e.g. Local value for values present in the XAML code, Style for the ones that come from a <Setter>, etc... (let's call it mValueSourceLogical)

So for data set properties these two values would be mValueSource = Default (so we keep the gray border around the property editor), and mValueSourceLogical = Local (means it's present in XAML code, clearing value is possible, enable the "Clear value" button, enable the logic in Property::ClearValue, etc...).