1. The DisplayMemberPath property is used for both the items into the DropDown portion of the control and the selected items. It would be great if you open a support ticket and send us a project demonstrating the issue that you describe.
2. The append functionality into SuggestAppendMode is applied when an item is highlighted. By default when writing text no item is highlighted. If you highlight an item using the arrow keys you will be able to see the append functionality.
3. This can be easily achieved by setting a TextBoxStyle to the RadAutoCompleteBox. Have in mind that you will have to base your style on the RadAutoCompleteTextBoxStyle, so you will have to merge the Telerik.Windows.Controls and Telerik.Windows.Control.Input resource files. Here is sample code:
There is also a sample project attached demonstrating how to achieve this.
the Telerik team