Now if you add a content to the panelbar the content inside will receive tab navigation only once due to the items control nature of the control.

The XAML team has recently reviewed the status of this issue and won't be addressing it in future releases.
In order to make the content (items) of a specific RadPanelBarItem (X)  focusable with tabKey, set TabNavigation="Local" on X.

RadPanelBaritem's expander arrow should not be visible when there are no subitems.

XAML Team has recently reviewed the status of this item. It is considered a design limitation of the RadPanelBar control which won't be addressed in future releases.
However, users will be able to use the ExpanderStyle property of the RadPaneBarItem (from Q1 2016 SP) in order to control the arrow's visibility.