The DragDropManager was introduced because some major flaws in the RadDragAndDropManager. Many of them related to drag and drop between different applications. Drag and Drop in nested scenarios etc.
As to the DropPreviewLine, it is an internal feature of the TreeView, not the RadDragAndDropManager. The TreeView internal Drag and Drop logic still depends on the RadDragAndDropManager, so disabling its internal drag drop logic disables the line as well.
At the moment the only way to use the Drop line is to use the RadDragAndDropManager.
As to the difference in the features of the both Drag Drop mechanisms, at the moment the one gap that exist between them is the lack of the DragArrowCue in the new manager.
Again we are sorry for the inconvenience caused, and are going to do our best to avoid such cases in the future!
All the best,
the Telerik team