I am simply cursed by the browser gods and basically give up and will wait for the problem to disappear - or not.
In my unmodified Standalone install of Vivaldi Stable I changed the active tab minimum width from the default to 150px and Vivaldi failed to start on next launch. I set that setting to default and made the change again but could not reproduce the crashed. A dead end.
That - and all the minor troubleshooting settings changes - is not something that should cause a crash. There is obviously something some incompatibility between Vivaldi and my hardware but my machine's drivers are up to date.
At least when Vivaldi is crashing like this the program asks me to send a crash report.
Thank you to everyone who spent their time trying to help me on this baffling issue. I'm sorry to have wasted your time and efforts to no end.