Argh. Solved!
Sheer minutes after posting I accidented a fix! Argh - after stumbling through registration, copying error texts, and ensuring that I've taken all reasonable steps already, and then finally posting too!
Here's what it took this fool: In NVIDIA Control Panel, I changed "Preferred graphics processor" from the default "Autoselect" to "High performance NVIDIA processor".
Hope this helps someone in the future.
Personally, I could not find anyone posting the same experience I had with Underrail (although there are cases out there for other games), so there's reason for me to hope.