Taxonomy autocomplete is empty because of access control for anonymous users
Changing the access settings didn't help because Drupal filters out items that are not accessible by the user currently viewing the view.  There are a number of ways you can deal with this:


Use Vocabulary Permissions Per Role which gives finer grained control of taxonomies
Use Page Manager (part of ctools) to override the taxonomy page either by redirecting somewhere else or making your own page in Page Manager using Panels.
As you suggest, use a redirection module (or Rules perhaps) to redirect users away from the page.  The data will be safe as long as the rule or redirection fires before anything is rendered.