I wrote an F# tool specifically for changing the primary display not too long ago. This is the code that calls the Win32 API to query the displays and set the primary one.

https://github.com/steinuil/SatouinDp/blob/master/DisplayDev...

One thing I never figured out was how to map the displays you get from EnumDisplayDevices to the names you see in the display settings, or how to list them in a consistent order.