Sorry for the double-post, but I think that I may have found the source of the problem, as well as a solution. I thus post it here for any who might come after:
In the data that I posted above, do you see where I listed the NVidia driver in use?
Well, it turns out that what I posted there was wrong. Not because I had misread anything, but because it looks like Ubuntu–again, I’m guessing as part of an update–decided to unilaterally and without informing me switch me over to the Nouveau driver!
Having stumbled upon this discovery, I proceeded to do a purge of NVidia elements (as a preparatory step), then installed a proprietary driver anew.
(For what it’s worth, I took the opportunity to switch over to the 550 driver; long story.)
And indeed, a quick test indicates that both Blender and GIMP are working as expected now!
(For the curious, I discovered this driver switch-over because I tried to play a game, and was concerned when performance was lower than it used to be. And yet the System Monitor didn’t show undue resource consumption. Quite what prompted me to think to look at my drivers I’m not sure–but I’m glad of the prompting!)