Going back to try this cracked the nut, thanks so much! I had tried to do exactly that but Preferences wouldn’t let me. I just remembered to option-click the disabled choice and it told me that the Mac had Filevault on that prevents auto-login, which it’s the default at setup time. When I turned Filevault off I was able to change the to automatic login, and all is working now 
How many people are still arguing in the forums about this when the answer is so simple, although really hidden as the interactions between the different Preference choices are totally opaque…