After struggling for the better part of a month to get Windows 7 installed on my older Intel NUC---I’m currently using a newer model each day as my primary PC, with Windows 10---I’m now able to move to this aging OS fulltime as I had originally planned. I think.
The issue, obvious in hindsight, was hardware compatibility: The Windows 7 installation media predates the USB 3.0 ports and M.2-based SSD storage used by the NUC, leading to all kinds of hilarity. Without USB 3.0 support---the NUC doesn’t even have a single USB 2.0 port---you can’t get past the opening screen of Setup. And without SSD support, Setup can’t “see” any internal storage, preventing Setup from continuing past that point.