How do I "see" / get selected nodes in the rendering window from C# / C++?
This really depends on your needs.
The MouseSelectNode() API function (in blade.cpp) may be what you're looking for. However, that won't catch drag-select operations; for that you'd need to look at SelectionManager, in the RegionSelectMode if statements.
If you need to know it for a specific node? There's a virtual SelectionCallback() function on Node objects. Do you need to know it for *any* change in node selection? SelectionManager::Select() is probably what you'd want to look at.