The problem is the type of object you're setting the current selection to. The individual parts and subassemblies you see in an assembly are ComponentOccurrence objects, not part or assembly documents. They reference part or assembly documents, but that's not what's in the assembly. It's also a ComponentOccurrence object that you can get the position information from. You use the Transformation property, which returns a matrix that contains the position and orientation information. There's a method on the Matrix object called GetCoordinateSystem that returns this information in an easier to use format.
http://help.autodesk.com/view/INVNTOR/2018/ENU/?guid=GUID-07042049-4E3C-48E3-8635-27B57353C4E5
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Brian EkinsInventor and Fusion 360 API Expert
Website/Blog:
https://EkinsSolutions.com