Ah. This makes sense now I think about it. I was wondering why a custom install of just Word and Excel required the same 2.57 GB of disk space as the express install everything option. But anyway.
Some spod at Microsith has claimed that the "Excel hangs when you look at it" issue can be cured by examining the entrails of a chicken jibbling with your default printer, but every time I tried to access "Devices & Printers" from the Control Panel the CP hung so badly it needed a reboot to restore it to life.
Removing and reinstalling Office did the trick and Excel was happy. And then Microsith installed 52 updates, and Excel broke again, and the conclusion we draw from this, which coincidentally is one of the fundamental principles of IngSoc, is that Microsith Spod speak with forked tongue.
Uninstalling all add-ins, and the 3rd-party "Print to PDF" application, and Office, and then reinstalling them, and allowing Microsith to apply its updates (four of which it decided "weren't needed"), and I can once more embolden the contents of a cell, and that's about four hours of my time I'd rather have wasted doing something useful, like getting to the bottom of why the in-game music player in ETS2 doesn't like ZZ Top.