Despite configuring three CB Plugins (Anti-Spam, Privacy, & Activity) to show in the Joomla Admin Menu, none do.

They'll be in the Community Builder dropdown to the left. Click it then click Plugin Management. That's where all plugin menu items are for compact display. If they're not visible or you don't want compact display then edit CBs admin module in Content > Administrator Modules (it's usually named something like "CB Admin Dropdown Menu") and under the Menus tab set "Plugin Menus" to "Show" (this is default enabled for new installs so should already be the case) and if you don't want compact then set "Compact" to "No" (this is default "Yes" for new installs).

More importantly, the CB Registration Form doesn't show the banner image

The registration header and footers are configured in your Default CB template within CB > Plugin Management under Layout > Registration. You can also configure profile view and profile edit layouts here too.

six CB fields do not show fields for new registrants to complete:

Field registration visibility is based entirely off it being published, set to display on registration, and the tab it's on also being published. If the fields are plugin field types then their plugin also must be public and published.