Re: GLX problems after failed rollback attempt
I had already purged nvidia-glx. Just now, I tried purging all remaining nvidia and glx packages, with the exception of the mesa package, and reinstalling 190.42. No dice. It just gave me this error again: failed to load GLX extensions (no compatible nVidia X driver found). That may not be verbatim; I was quoting from memory. I'll give the beta driver a chance now, and hope that that works.
Turns out there actually wasn't a problem with the GLX extensions in the first place. I accidentally booted normal Kubuntu when I meant to boot recovery mode, and much to my surprise, it worked. I had been trying to launch the x-server from the recovery CLI rather than rebooting, but apparently that's not a good idea. Now, back to trying to get the driver to cooperate.