Due to a popular feature request to support ReadOnly mode for custom editors, we have internal logic that binds their IsEnabled/ReadOnly. Thus, its previous value gets cleared. A simple workaround is to place the editor in some panel (panels does not support IsEnabled).
As for the other inquiries of yours, editors' DataContext is explicitly set to RadPropertyGrid.Item at runtime. So that, it is expected to set PropertyDefinition's Binding.Path to a property of the Item's type.
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