So it seems that if you run on battery power, closing the lid prevents the computer from staying awake.
What I simply want is to run the computer on battery power at the external monitor resolution, I don't care if the built-in display is open, closed, on or off.
Download caffeine or InsomniaX, because clamshell mode(external display with closed lid) is only meant to work when battery powered.
Defining a fake built-in resolution equal to that of the external monitor with SwitchResX allows mirroring at the bigger resolution (the built-in goes black, but I think the backlight is still on).
So it seems that if you run on battery power, closing the lid prevents the computer from staying awake.
What I simply want is to run the computer on battery power at the external monitor resolution, I don't care if the built-in display is open, closed, on or off.
Why don't you just turn disable mirror displays, set the correct resolution on external monitor, and then drag the menubar icon (in displays manager) to the external monitor to make it primary?
That doesn't transfer the windows from the built-in to the external, and is a drag to do every time.
That also leaves the built-in switched on.
This is better: close MBP so that it goes to sleep, plug USB device so that it wakes up, open lid and built-in screen stays switched off.
I still haven't tried this sequence on batteries.