1) You can set MouseSelectionMode property of the RadMap to "RaiseEvent". Then you can handle SelectionRectangleChanged event. If you use Shift + Mouse Drag you will receive selection rectangle on top of the map. When you release the muse, the event will be fired with Rect event arg marking the selected area. Check the ItemsSelection article for VisualizationLayer
2) Using the MapMouseClick event you can add in code element (puishpin, rectangle, polyline, etc) on the clicked map location.
3) RadMap can read such big files but you should have in mind that placing thousands of objects can lead to performance issues. In such scenarios you can refer to Visutualization
Telerik by Progress
Do you need help with upgrading your AJAX, WPF or WinForms project? Check the Telerik API Analyzer
and share your thoughts.