....the problems returned and I have now re-installed Mojave without the two updates. I have no way of knowing if the security update is the problem or if it's the OS supplemental update, which introduces the newer version of Safari...
The Software panel now has:
Security Update 2020-005 10.14.6
Release date September 10.14.6
Safari 14.0.1
The macOS Supplemental Update 10.14.6 included installation of Safari 14.0. I wonder if this addresses my quandary "is the problem the Security update or Safari update?" Maybe Safari was the problem and Safari 14.0.1 is the fix.