Okay, I’ve added the above code to wp-config.php. At this point, as single site admin I can see URE under Users and under Settings, but not under Plugins. (I can see all as network admin on this single site.)
My host, WPEngine, occasionally monitors and strips code from wp-config.php, I believe, so above may not be a lasting solution.
Btw, it would be important, at least in our current scheme, for the Multisite tab to be accessed by admins and apply to the site only: the specific setting (which I know is risky) is “Enable “unfiltered_html” capability,” as certain students (who are admins of their own sites) need to embed concept map code from another provider, and this requires <object>…</object> code. If this will not be possible, then super-admin would need to be able to enable this capability to admins on selected site only.