I understand what you are doing. I've done some searching for an answer to this but other than experimenting with Vista VGA drivers I've not come across many viable solutions.
Please let us know if you find something that will work.
Did you actually disable or uninstall the onboard GPU
if you uninstalled it you could try reinstalling and then just disabling it and disable in services.
This is one method I found to re-enable sleep in the Shut down menu not sure if it will work for you but may be worth a try.
From Control Panel, Power options, Change Plan Settings, Change Advanced Settings, click on + next to USB Settings, click on + again next to USB selective suspend settings and Disable the setting.