Xorg blank screen - after installing nVIDIA drivers v361/v367 (and card not used for display)

I have a problem which is similar, but may not the same, as in several other questions asked here. The closest seems to be:

Can't get nVidia drivers working with 16.04, logs out right after login

Here's my scenario:

• Kubuntu 16.04 64-bit, freshly installed.
• Desktop machine with an on-board (on-chip) Intel display, and a discrete nVIDIA GTX 650 Ti - not used for display, just for computation.
• Installing nVIDIA driver version either 361.62 or 367.35 (both with same result)
• No 'Optimus', whatever that is.

Well, the driver installation works (but it begins with a complaint about a preinstall script failing); and on reboot, the nVIDIA driver(s) appears with lsmod. Also, CUDA code runs successfully so no problem there.

However... when booting the VT in which X is supposed to show me a login screen is blank. The machine doesn't hang (I can work on other VTs), nor does X crash. The errors I get in /var/log/Xorg.0.log are:

[    15.618] (EE) NOUVEAU(G0): [XvMC] Failed to initialize extension.
[    15.642] (EE) Failed to initialize GLX extension (Compatible NVIDIA X driver not found)

If I uninstall the nVIDIA driver, X works properly again (will quote some log messages here soon).

The solution for the similar question does not work for me. So my question is:

How can I have the nVIDIA drivers installed and the X server work, considering the fact that I do not use the nVIDIA card for display at all?

Obviously, a solution which prevents X from "noticing" the nVIDIA card would work well for me.

FYI - I'm also having a variant of this problem on Debian Stretch, so a non-Ubuntu-specific solution would be swell if you have one.