In order to get the Nodes count you can still use the Embedded TreeView control of the DropDownTree, but at server-side. In addition, regarding the SelectedIndex that you need to set - you can access the NodesCollection (index-based) of the Embedded TreeView and use the SyncEmbeddedTree method.
In the attachment you can find a simple example demonstrating the above suggested approaches.
Hope this would help.
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