I don't understand what you are trying to do.
If you dim the laptops monitor so that you can't see it or close the lid, you don't have the use of the laptops screen. So there is no practical difference from doing a monitor switch using the F7 key.
* At times, I want to use both screens in extended mode.
* At times, I want to dim/turn off the laptop screen only.
* Fn+F7 results in moving windows from one screen to the other, which I don't want.
* Closing the lid and reopening it does similar to Fn+F7.
* I just want to dim the laptop, while leaving all windows on both screens unmoved, then later fire up the laptop screen, with all windows in original positions. I can do this if using two external monitors only; just physically turn off one monitor, then turn it on again later. Can't seem to get this same result if one of the screens is the laptop itself.