W530 Optimus driver won't load

My workplace issued me a Thinkpad W530 ( while ago), and the Optimus drivers have always just plain refused to load.

I recently had the motherboard replaced because the laptop was powering off completely at random, even when on AC power.
The IT guys said they tested it with 3DMark, and it worked fine. Once I got the thing back, I seem to recall the driver loading exactly once... but now, it's gone back to not loading.

Currently, the Quadro K2000M shows Code 43 in Device Manager. If I disable and re-enable the device, the entire desktop dies -- it drops to a black screen with only a cursor and a lot of disk activity.
After a hard-reset, Event Viewer shows that logonui.exe was repeatedly crashing in "nvumdshimx.dll".

Similarly, the nvidia control panel won't open. It doesn't even show an error -- instead, I have to look in Event Viewer to see that nvcplui.exe crashed.
(In the past, I've even seen the driver load, but then the NVIDIA control panel complain that "you are not using a display connected to an NVIDIA graphics processor". Well, duh... that's the very definition of Optimus!)

I've even tried fresh-installing the Windows 10 preview, to see if my Windows 8.1 was just broken -- nope, same behavior there, both with the Lenovo driver and the Windows Update drivers.

Oddly, the nvidia driver actually loads properly in discrete-only mode, which tells me that the GPU is alive.
Discrete-only mode has various tradeoffs, though: lower battery life, more fan activity, poorer Linux support (my primary work environment), and an excruciatingly slow EFI framebuffer.
EDIT: Oh, and discrete-only mode also has this insanely distracting "adaptive backlight" (change backlight level as screen contents change) feature that you can only disable by going into the registry to unhide the setting first.

How can I get the driver to load, instead of just killing everything? Has anyone else seen this behavior?

 

May I please know where this registry setting is? Thanks.