Subject: Solr Admin UI with restricted authorization

You can restrict the admin UI by limiting access using the authorization plugin. I would though not give access to end users for the admin UI.  A good practice is to create your own web application running on a dedicated server that manages all the authentication / authorization and provides a UI for the end users. Then you can manage access in a dedicated enterprise directory and also implement single sign-on etc.
You cannot manage users in the admin UI and you will see it gets very cumbersome with many different users.

Also from a usability point of view the admin UI does not make sense for end users.