copied an existing 2013 site and upgraded it to 2018 and I am playing with BootstrapResponsive theme to see what it can do.

Old 2013 site had modified theme with manual menus as HTML blocks on pages. Top menu of old site is Javascript within this theme (site originates back from BVC4.x)

I've switched over to a copy of BootstrapResponsive theme. Have gone thru all main categories and 2nd level categories and checked off on them to show in top menu. But nothing is showing up at all here.

On another new site, with a fresh load of 2017 (upgraded to 2018) f, top menu works perfect

Again, i know it has to be something simple. A switch in admin settings? I've looked everywhere and do not see any errors, display:none code, old JS code that could be rendering the auto-modules inactive, etc.....

This is a content change, which could adversely affect an existing customer's site, which is why this is not part of the upgrade. You will find many things like this when trying to use the Bootstrap (or the older Foundation 4) responsive theme with an old site/theme. The best thing to do is to use the demo store (or a fresh installation) as a reference for the necessary Content Columns (and their Content Blocks) as well as other site settings.