How can I display only Cases without Case Managers using Drupal Views
You are close.
Add a Contact A relationship and use  CiviCRM Relationship (starting from contact A).

Add both Display Names fields.

In filters, add CiviCRM Relationships: Contact ID B and set it to empty.

Here is a copy of a view of CiviCRM Cases.


And a pic of the filter:


Update to solve multiple cases:

To solve the folks with multiple case types issue, I created this page view but filtered for only one case type.  I then added attachments to the view page for all other case types.