When a control is nested in a RadPanelItem, it is wrapped in another RadPanelBarItem. The DataContext of the second level item is not inherited correctly. As a workaround, you can bind directly to the element which holds the DataContext with an ElementName binding.
Expose a property to easily control the expander indicator style and visibility. From Q1 2016 SP users will able to use the ExpanderStyle property of the RadpanelBarItem in order to style the arrow expander.