What determines the suite of Case Roles presented for assignment on the CIVICRM Webform Case Tab?
Come to find out that the role not being available to map was because the relationship name and label in the database did not match (i.e. name_b_a was "Third cousin once removed" but the label_b_a was "Distant cousin").  It seems like when the relationships were originally created we had the ability to set a different name and label - we're guessing this was in an earlier version of CiviCRM as currently the only thing you can add/edit directly in CIvi is the label. 

Once the name field was revised to match the label, the role was available for selection in the Cases > case roles section.