I'm not convinced that you have a real problem.
If you could provide a screenshot - particularly the System Information showing that there's two screens registered by the system.
Graphics & Displays tab should show information about the graphics card, and also information about the display. That's actually two separate items, so is that what you see? Or, does the tab ALSO show information for a second display?
Or, a picture of the display after you change the settings in Displays pane? (Best for Display!!!), and then restart to see if that setting holds.
And, your next step should be to call AppleCare. They should be able to walk you through some steps. Tell them about the reinstall, and what your screens look like. Tell them that you tried a PRAM reset (it may not have HELPED you, but it did, in fact, give you another item to tell AppleCare)